Effect of gear oil viscosity on transmission efficiency (Metro owners take note)
 

This post is really about 2 things:

1. Specifically, I believe the transmission oil weight called for in the Suzukiclone owner's manual is incorrect, and
2. An experiment that starkly illustrates the magnitude of energy losses in a manual transmission comparing "thick" vs. "thin" oils.

If you don't have a Suzukiclone, just skip Part 1 and scroll down to Part 2 for the interesting viscosity comparison results.


http://www.austinev.org/evalbum/imgm...pg&w=300&h=225

Part 1.
------

Why I believe the Suzukiclone owner's manual recommended transmission oil weight is wrong

An acquaintance in British Columbia converted a 1987 Suzuki Forsa to electric drive, and was concerned that he was seeing a lot of energy loss in the drivetrain (based on the amount of energy required to spin the wheels on jack stands).

He mentioned to me that he was thinking of going to synthetic oil in the transaxle, and I told him I believe the specification in the owner's manual (calling for 75w90 gear oil) is incorrect.

I came to this conclusion after having changed the transmission oil in my own 2 Suzukiclones. The first one I changed to 75w90 synthetic soon after buying it. I noticed afterward (but wasn't certian) that it seemed to increase the amount of "grinding" of synchros in quick gear changes - a common complaint about the transmissions in these cars.

When I got Firefly #2 (the Blackfly) with just 2,000 km on the odometer, I also thought I should change its transmission oil, because it sat parked for 7 years. But when I did this, I was careful to pay much closer attention to the viscosity of the fluid I drained from it - and I saw that it was obviously MUCH thinner than the 75w90 oil called for in the owner's manual. Note this was unquestionably the factory original tranny oil that was drained from the car.

A bit of investigation on teamswift.net uncovered several people recommending GM's AC Delco Synchromesh brand of semi-synthetic manual transmission oil (it doesn't have a viscosity/weight listed on the packaging). I bought some, but before pouring it in the car, I did a few tests to compare the various viscositis of the oils I had:

• I did a simple timed "pour test" of a measured amount through a cotton filter, and saw that the AC Delco Synchromesh lube was actually closest to 5w30 engine oil in viscosity (based on the time to drain a measured amount through the filter).
  .
• I also pour tested the factory original oil that I drained from the Blackfly (Firefly #2). It also poured through the filter at the same rate as the Synchromesh oil and 5w30 engine oil. Hmm!

This told me that the manual-recommended 75w90 synth oil I put in Firefly #1 was too heavy. Fortunately, I owned both cars simultaneously for a couple of weeks after getting the Blackfly (before selling #1). So I was able to do another test:

• my next test was to drain the 75w90 synth oil from Firefly #1 and replace it with the "old", thinner, factory original oil I drained from Firefly #2.
  .
• After doing this, the amount of "synchro grinding" immediately reduced.

So I returned the 75w90 synthetic gear oil I had bought for Firefly #2 to the store, and instead added the GM Synchromesh oil to that car. I have had almost no trouble with its synchros since then. I also "moved" that fluid from the original transmission to the used "taller" transmission I installed later on. Still little synchro trouble to speak of, even though the used transmission has 160,000 more km on it than the car I put it in.

From all of this, I came to the conclusion that the 75w90 recommendation in the owners manual is incorrect. It's too heavy.

Part 2
------

An experiment comparing energy losses from "thick" vs. "thin" transmission oils

Written by Roger - electric Forsa owner.
http://tech.groups.yahoo.com/group/ev/message/35167
Apr 12, 2007

RE: Sprint/Metro Drivetrain losses

I had written:

> The tranny is full of fresh 75W90 gear oil, per the manual,
> however, I've been advised that the manual is wrong and a GM
> semi-synthetic lube is what should be in there. I'm
> skeptical that the lube alone could be responsible for this
> sort of loss, but since an oil change is easier than
> rebuilding the tranny, I'll try that first ;^>

As it turns out, it seems (from checking a partial bottle remaining in
the garage) that the tranny is actually full of 80W90, not 75W90 as I
originally wrote.

I put the car on stands again (under the frame with the suspension at
full droop since the suspension angle didn't seem to affect energy
consumption in my earlier tests and I feel better with the stands under
the frame when I'm going to be getting under the car).

All following energy consumption observations are based on battery pack
voltage and current as reported by my E-Meter.

I spun the wheels in 2nd for a few minutes to warm things up first, then
measured energy use in each gear at 40kph (except 1st, which was
measured at 30kph):

5th 23.4A @ 123.5V (116.3Wh/mi or 72.2Wh/km)
4th 22.9A @ 123.0V (113.4Wh/mi or 70.4Wh/km)
3rd 25.5A @ 123.0V (126.2Wh/mi or 78.4Wh/km)
2nd 30.8A @ 121.5V (150.6Wh/mi or 93.6Wh/km)
1st 37.5A @ 121.5V (244.5Wh/mi or 151.9Wh/km)

After draining the old fluid, I poured in a half litre or so of Varsol
and spun the wheels for a minute or two to flush things out:

2nd 22.5A @ 123.0V (111.4Wh/mi or 69.2Wh/km)

This was then drained, and the tranny filled with 2.5 litres of the
recommended AC Delco Synchromesh fluid (p/n 89021808, IIRC):

5th 16.1A @ 123.0V (42kph, 75.9Wh/mi or 47.2Wh/km) (34.7% lower)
4th 17.0A @ 122.5V (42kph, 79.8Wh/mi or 49.6Wh/km) (29.6% lower)
3rd 19.7A @ 120.0V (42.5-43kph, 89.6Wh/mi or 55.6Wh/km) (29.0% lower)
2nd 22.4A @ 120.0V (39.5-40kph, 108.2Wh/mi or 67.2Wh/km) (28.9% lower)
1st 37.7A @ 118.0V (39kph, 183.7Wh/mi or 114.1Wh/km) (24.9% lower)

So, going to the [GM Synchrmesh] fluid definitely seems like a step in the
right direction, however, not nearly as large a step as is required http://www.gassavers.org/images/smilies/frown.gif

On the plus side, the 3rd-2nd downshift "crunch" is gone, which had been
one of the touted benefits of using this tranny fluid.

So, some progress, but the hunt for better efficiency continues...

Cheers,

Roger.

---

Just a note: the energy savings seen from the less viscous gear oil doesn't translate directly to an equal decrease in total fuel consumption (or electricity use, in this case). Only the energy difference required to spin the wheels on jack stands was measured - not the total difference to actually drive the vehicle (with all its other associated losses factored in).

vw recommends 80-90 also. i been using john deere hygard(hydraulic oil) for over a year now, with smoother shifts and no ill affects so far.

Metro, I've got two quarts of Amsoil Manual Synchromesh TF, full synthetic, 5W-30. You or Roger can have it if you want to give it a try. My Blazer's engine blew up before I got it installed, and now I'm driving an automatic. PM me if you're interested.

An interesting comparison.
Reductions of about 30% on average are well worth having.

As an aside the Borg Warner T5 runs more efficiently and certainly more smoothly on ATF than any specialist gear oil in my limited experience.

Pete.

75W90 is equivalent to 5w30 I believe. Transmission fluid and Engine oil use two different viscosity ratings. I don't know if it is common knowledge around here, but I remember being a bit mislead when I first read about transmission fluid.

I'm not refuting your results, nor can I explain why the change to synthetic brought such great results (other than less viscosity at lower temperatures).

I do know that transmission fluids are rated based on protection levels (1-5, but I've forgotten the exact name), therefore lower viscosity oils can essentially be equivalent to those much thicker. The lowest viscosity MTF I've seen is VW G52(which has a protection rating of 4), but ATF's are down there also.

G52 is not synthetic so it only has its rated viscosity @ ~100C. I believe VW also makes another extremely lightweight synthetic called G70 or something along those lines. Last I read about it, it was extremely expensive ($14/quart or something equally ridiculous).

I say run as low a viscosity as you are comfortable with and drive like you don't have synchros. Revmatch/double clutch...hard to master but cuts down on synchro wear/grinding.

Just imagine doing that @ 30mph... :)
http://www.youtube.com/watch?v=0j-3xIZK-Bk

- LostCause

Nice pedal work. Double clutchin', heel-n-toe. Squeelin' tires. Sweet.

I don't go for the acceleration anymore, but I still pick my lines on the ramps. I'll have some guy on my tail 'cause I'm only doing 55. Then, when we hit the ramp, I'll maintain my velocity while he hits the brakes for a tight curve and ends up 200yds behind me while I'm merging.

Oh yeah, viscosity. And, um, low viscosity gear oil helps.

What about a RWD car with a separate differential case? BMW recommends ATF for my 5 speed manual gear box and 80w90 for the differential. Would it be crazy to try running synthetic ATF in the differential and the gearbox? In most FWD cars the ATF or transmission fluid lubricates both the gears and the differential. Thoughts?

thanks
Justin

Justin,
Personally I would follow the BMW guidelines.
The FWD cars using ATF have diffs designed to run on it.

Cheers , Pete.

Yeah, I think ur right. After reading further about transmissions that can run either ATF or gear oil, mixing traces of gear oil with ATF can cause the oil to clump up and destroy the transmission (or differential in this case). I think I'll just switch them both to synthetic fluids and leave it at that.

That's the first I've read about dual viscosity rating systems for gear vs. engine oil. Can you point to a source about that?

I can tell you that 75w90 gear oil is absolutely not the same viscosity as 5w30 engine oil, because I compared them by pour testing at the same temperature through a filter. There's a dramatic difference.

75w90 gear oil is in the range of 10w40 to 20w50. The difference is in the additive package. The gear oil is designed for extreme pressure while engine oil is designed to withstand extreme temperature.
http://www.bobistheoilguy.com/images/visc.jpg

Thanks for posting that, tjts1.

There is a Standard
 

Link for info on Motor Oil. www.api.org.

http://new.api.org/certifications/en...lGuide2006.pdf

http://www.MetroMPG.com/

I run a Mazda MX5 (Miata) gearbox using gearoil mixed with20% ATF. I was advised to do this by a bloke who is the chief mechanic (has 50 plus staff) for a GM franchise. It immediately reduced crunching selector noise between first and second. He claims that putting ATF in gear is routinely done in his business to reduce noise and to make gearboxes less "sticky".

I wonder if I should cease this practise as I do not want to blow up my gearbox?

I did that to an old sube..a 1987. same thing. fluid listed was bizarre thick. Tried synthetic against advice for the year (1987 didn't know what synthetic is) and as complicated as a synchrod, differential mounted dual range one piece tranny is...the synthetic really came through after a few months..dramatically. The key to winning is the wait...

I am going to seek that gm stuff...great test and thanks.

http://i75.photobucket.com/albums/i3...TransFluid.jpg
http://i75.photobucket.com/albums/i3...esh_bottle.jpg
http://i75.photobucket.com/albums/i3...tf/Syntrax.jpg

GM Synchromesh Fluid (Made by Penzoil/Quaker State) is the best non-syn tranny fluid you can buy for a FWD Transaxle car. Civic dudes love it , made my other 2 Civic trannys shift like butter.

I had a Escort GT (Mazda 323) which called for ATF so I ran Mobil 1 ATF in the Manual tranny, shifted like crap, I put the Quaker State Synchromesh in and it shifted like BUTTER!

Guys who have DSM AWD Turbo car like Eagle Talons and Mitsubishi Eclipses swear by the Synchromesh fluid as it make this trannys shift so much better and they are VERY prone to grinding.

IMO I would run Synchromesh in any Transmission that specs ATF or Motor Oil for fluid

I had the GM synchromesh fluid put in my Tracker transmission on its first change. These tranny's tend to grind the 1st to 2nd shift and that fluid was supposed to help. I have to say I didn't notice better shifting at the time. So for the next change I just went with cheap conventional fluid and noticed that it seems to make the 1 to 2 shift take slightly more effort but its less likely to grind.
I now just wait for the rpms to match before making the shift and it works fine. I have no stop and go traffic so I make the 1-2 shift about 10 times on the way to work so no big deal.

As for friction, I don't think the dealer put in synthetic fluid in the difs or transfer case but I think I am getting more rolling resistence after I put in all conventional oils. It amazes me how much better my neon rolls than the tracker and that extra friction only costs me 6 or 7 mpg. Next change I'm going all synthetic and thin as possible so I'll have to do an A-B comparison on rolling resistence. I imagine in winter the savings should be significant as you can feel all the fluid warm up and the resistence decrease!
Ian

When the dealership first replaced the tranny fluid in my '99 Metro, it felt like I was rowing a spoon through peanut butter. (Two further synchro failures happened within 30,000 miles of each other following that.) I guess I'll ask before just trusting the manual from now on.

Some Gear Lube and ATF Data
 

I pulled this together for a VW TDI forum. Last update was Oct '07. Most data comes from manufacturer spec sheets, but has been validated and/or expanded with oil analysis. Sorry...it's heavily weighted to Euro and VW fluids.

There's been a steady shift from 80-90 gear lubes to fluids that are basically automatic transmission fluid with some extreme pressure additives in order to improve corporate average fuel economy...so manufacturers can sell more trucks... You can see that the lightest fluids are off the chart...

When it comes to improving fuel economy thru changing fluids, the differential is the largest factor, with tranny second and engine last.

Changing from petroleum to synthetic - even with the same viscosity - can make a significant difference. For example - a class 8 truck (semi) tractor with dual drive axles, when moving from petroleum fluids to real synthetics, can gain 8.3%. 5.25% is the differentials, about .5% is the engine.

I did some highway testing with my old VW diesel - moving from the factory 75W-90 synthetic, to a synthetic synchromesh fluid, down to VW G52, and back up. The move from 75W-90 to the really thin G52 was worth 1.8mpg. In the end I moved back to 75W-90 to keep my 390,000 mile transmission a bit happier.

VI is viscosity index and speaks to viscosity stability as temperatures change. Higher is more resistant to change with temperature.

Transmission fluid viscosity is normally rated at 40C, while engine oils are rated at 100C. And, as already pointed out, the viscosity scales for gear oil and engine oils are different.

Viscosities are in cSt - centistokes - and are essentially a timed flow thru a cup with a hole in it.

(Manual Tranny Fluids)

VI Vis@40C Vis@100C
128 159.0 18.3 = AMSOIL CTL SAE 50 Powershift GL-1
..............16.7 = Motul MOTYLGEAR 75-90 GL-4/-5
..............15.6 = VW G50/G51 GL-4
185 90.0 15.6 = Redline MT-90 75-90 GL-4
..............15.2 = Mobil 1 Synthetic 75W-90 GL-5
..............15.2 = Motul Gear 300 75-90 GL-4/-5
..............15.0 = Elf Tranself Synthese FE 75-90 GL-4/-5
132 116.0 14.9 = AMSOIL AGL 80W-90 GL-5
177 84.5 14.7 = AMSOIL MTG 75-90 GL-4
..... 76.6 14.2 = VW G052-911
133 76.2 11.0 = AMSOIL CTJ SAE 30 Powershift GL-1
183 56.2 10.6 = Redline MTL 70-80 GL-4
194 47.1 9.6 = AMSOIL MTF Synchromesh Trans fluid (GM/Chrysler) GL-?
208 41.6 9.1 = Penzoil Synchromesh trans fluid GL-?
198 34.0 7.5 = Redline D4 ATF Dexron III / Mercon / API GL-4
138 40.5 7.1 = AMSOIL CTG SAE 10W Powershift GL-1
..... 31.2 6.5 = VW G-052-171-A2 GL-?
..... 35.1 6.4 = VW G-055-726-A2 GL-?
..............6.3 = VW G52 (part numbers G052726A2 / G05272601) GL-?

(Automatic Tranny Fluids - except for Redline D4 dual-use)

VI Vis@40C Vis@100C
.............8.3 = Honda CVT Fluid
.............7.6 = Mobil 1 Synthetic ATF

198 33.5 7.5 = Redline D4 ATF Dexron III / Mercon / API GL-4
.............7.4 = Mobil 1 Synthetic Dexron/Mercon
197 32.5 7.2 = Redline Synthetic ATF Dexron II / Mercon
.............7.1 = Mobil 1 Synthetic Multi-vehicle ATF
138 40.5 7.1 = AMSOIL Ford type F auto trans fluid
168 37.4 6.8 = AMSOIL Automatic Transmission Fluid ATF
..........5.5-6 = Ford Mercon SP

This is great info - thanks for posting. I think I'm going to split off (copy) your post into a dedicated thread as well, since this one seems to be addressed to Metro owners.

Dedicated thread is: http://ecomodder.com/forum/showthrea...ands-8047.html

I can tell you that the manual transmissions in the late 70s/80s and into the early 90s VWs seem to have the same problems with syncro grinds/etc (Transmission model 02O or 020). Book spec calls for 75w90 also, when I drained my gear oil it was a LOT thinner (like your description of 10w30). Many of us in the VW world have either gone the way of Redline MTL (or similar) or several like myself have gone the Syncromesh route. I run Sunoco Syncromesh in my daily, and I have Royal Purple Syncromesh to run in my race cars. We have ALL noted that the infamous syncro grinds on shifting have either disappeared all together or have greatly reduced (likely due to already permanent damage to the syncro) running Syncromesh.

My ONLY qualm about it is DO NOT buy it directly from a GM dealership.... I got raked over the perverbial coals on its cost

The other thing to consider: GM syncromesh is "semi-synthetic" , if I recall. So its viscosity isn't as stable as temperatures change as a full synth. (Meaning greater efficiency hit for those who experience truly cold temperatures part of the year.)

I've got GM Synchromesh (full retail pop! :)) in my car now, but am planning to go to a full synth of the same or lighter weight, soon.

Just trying to decide which, and then seeing if my choice is even available in my small city.

ahhh.... interesting... I thought ALL syncromesh was atleast a semi, if not a full-synthetic. Is there a "traditional oil" syncromesh?

I bought the Royal Purple stuff from my local PepBoys :rolleyes:

Someone wanna try ATF+4 in a manual? It has friction modifiers in it, and it's rated for use in ALL Chrysler auto transmissions, regardless of age. This might really help an Eco-modder or two pick up some more mippigs?

auto tranny fluid?

Yep, many manual transmissions call for ATF. I think most Ford transmissions actually call for it, but you can use engine oil as well.

Is it Synthetic?
 

Some scattered thoughts...

There are three basic categories of base oil on the streets. Group I and II are refined petroleum. Group III is refined petroleum that has also been dewaxed (improves low-temp performance) and high-pressure hydrogen processed to improve high temperature performance. Group III can be called 'synthetic' (legal definition) in the Americas but no where else in the world. Group IV is man-made synthetic base oil - PAO. Group V is an 'all others' group and contains petroleum and synthetic products - from Vaseline petroleum jelly to the man-made esters used as a component in lubricating oils.

Most of the products on the North American market - the 'synthetics' and 'synthetic blends' are made with Group III base stock. All of AMSOIL's products (with the exception of the XL products) is Group IV/PAO based. RedLine products are primarily ester based. (Both companies use a combination of PAO and ester.). Mobil 1 products are primarily PAO-based, but some also contain some amount of Group III (legally synthetic, but technically a synthetic blend because of the petroleum-derived content.)

Gear teeth are kept apart by a combination of fluid viscosity and the extreme pressure components in the oil's formula. Moving to a lower viscosity product can increase efficiency but can also result in increased wear rates.

As viscosity thins, the anti-wear additives must be improved to maintain the same low wear rates. Keeping wear in check is important because it's a geometric progression - one piece of steel or grain of sand in a gear box can scrape a pair of pieces from two gear teeth. The three pieces can generate 6 new pieces, the current 9 pieces can generate 18...and away we go.

It's best to match both the viscosity and the performance rating (such as GL-4) when selecting a replacement fluid.

If the transmission is filled with 75W-90 GL-4 petroleum, move to 75W-90 synthetic (AMSOIL, Redline, Mobil 1, or a European product) that also has the GL-4 (or similar OEM rating). Next move might be to something in the Synchromesh arena.

Driver technique and aerodynamics are better first targets - efficiency gains from lighter lubes are measurable but small, and if taken too far can lead to unhappy synchronizers and/or increased wear.

GL-5 is for differentials; GL-4 is for transmissions with 'yellow metals' and synchronizers. The higher levels of extreme pressure additives in GL-5 fluids can destroy brass, bronz, and copper in a manual transmission. Some 'real' synthetics can have dual rated GL-4/GL-5 products because the company found that the heat-activated GL-5 additives don't activate in a cooler running transmission. But it's best to stick with GL-4 or appropriate OEM rating for a synchronized transmission.

Please do not add anything to any finished lubricating oil - including transmission, engine, or differential. At best you're only out a bit of money. At worst one can lose their component warranty, oil warranty, have bearing wear and/or increased rust/corrosion. Lubricating oils are designed and tested specifically for the intended application - don't upset the sensitive chemical balance.

Andy

'97 VW Passat TDI wagon 391,000 miles

The Right Oil For TransAxle
 

Here's from mya Mazda Astina Service book,
boaut thspec for transaxle oil:
TransAxle Oil picture by herbid - Photobucket

synthetics
 

cold pumpability testing of full synthetics reveals ease of flow found only in lower viscosity standard oils.
Since there are 3-criteria for oil viscosity ratings,not just one,oil makers are forbidden by law to advertise the fact that their oil might flow like 0-wt.

many chrysler mtx's call for atf, but, and this is a big but, it is used to save money. The synchros will wear faster with it, and it will not shift as easy as the correct mopar fluid ($16 a quart) or mobil one 10w30 high mileage. Its very important to use mobil one 10w30 high mileage if you choose to use engine oil, especially in a neon, as the synchros are made of a material called fiberide, and according to oil analysis, only those 2 oils (the mopar stuff and mobil one stuff) have enough zdp to prevent premature wear to the synchros. Mobil one is 99% the same as the mopar stuff.

ive used both royal purple and penzoil synchromesh oils, and neither shifted as smooth as the correct mopar fluid or the mobil one oil

Cool. I'm not personally concerned about synchros, cuz I don't use the clutch anyway, but yeah.

ya, i dont use them much either, 90% of my driving is freeway, but just wanted to make sure i posted in case anyone switches to atf then has problems. Its not really a immediate thing, more like a if you plan on putting 300,000 miles on the transmission thing

regular 75-90 gear oil on the other hand can destroy neon synchros in 5000 miles tho, thanks valvoline oil changers

Been using ATF in a 1967 4 speed manual Ford for years now. Originally had some heavy oil, might have been as much as 140W. It was getting very hard to shift into 2nd gear. Used to be fine, but then the shift to 2nd became stiff when the car was cold. Got worse, so that 2nd was hard to shift into when warmed up, and almost impossible when cold. Was taking serious force on the gear shift to get it in 2nd.

We switched to ATF, and 2nd gear has been fine ever since. Not sure how many miles we have put on it since switching, but at least 20k.

One thing about differential oil: has to be specially formulated for hypoid gears. Lot of pressure on those. ATF won't cut it.

Does any1 know what the viscosity of lucas oil additive is because it says if u put all lucas in ur differential it will iliminate wear and if it's about a 65w when ur supposed to use 90w i'd say it's definently worth it????

this incorrect oil problem ive dealt with in Isuzu's too.


many fast lube places for some strange reason:confused:
have it in there computer that they take 80W-90 gear oil in the trans, when its really needs to be 5W-30 engine oil

in one to five years the bearings are shot:eek:

The high viscosity ensures transfer of lubricant throughout the gear train. This is necessary since the devices needing this heavy oil do not have pumps for transferring the oil with only a portion of the lowermost gears bathed in an oil sump.

Any final conclusions on which gear oil to best maximize fe without risking damage?

Nope. I ended up just staying with the GM Synchromesh oil.

The original Lucas Oil Stabilizer is quite thick, well over 90 SAE weight, Lucas Synthetic Oil Stabilizer is thinner than the original but still thicker than 90 weight. I know because I've put both in manual transmissions that required 75w90 oil. But they're not really an oil, they're a slick sticky substance, I believe they coat the moving parts when added to oil. I put original into my Corolla, which required 5w30 engine and 75w90 gear, mixed with synthetic oil and I swear it made the car slower to start when cold in the winter (nothing serious, just a few more turns, still started reliably), but Lucas advises against using original in lighter oil. So with my Yaris I used the lighter synthetic, but used less than the 20% called for on the label, I think around 10% in the engine and 10-15% in the transmission and haven't noticed any problems, even in cold weather.

Racers and off-roaders seem to like Lucas Oil Stabilizer, but then again, they push engines and drivetrains to the max, unlike us daily drivers.

Also the fact the engine and gear oil use different weight scales makes sense. 75w90 gear oil seemed way thinner than what a 75w90 engine oil would be, but it's really the same weight as 10w40-50 engine oil. And about using thinner fluid than called for in a transmission seems like a bad idea, unless there's evidence to back up its use, like the Suzukis and Synchromesh. So, I'd say, unless there's a problem with your transmission and you think it might be too thick fluid, stick with the recommended weight. A new transmission, or a rebuild, costs a lot more than the few more miles you can squeeze out of a tank with lighter fluid. Damage or wear may not be apparent right away, it might take years and thousands of miles for problems to show up.