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Old 10-05-2011, 05:46 PM   #1 (permalink)
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Thinner differential oil for RWD car? ('06 Miata)

I recently sold my '06 Jetta TDI DSG (127k mi) and bought an '06 Miata Touring 5-speed (19k mi). The Jetta's DSG started slipping, and it seemed to be in danger of needing a very expensive replacement transmission. I've wanted a Miata for years, and now I have one!

On the Ecomodder forum, I've seen lots of really interesting experiments and experiences with manual transmission oil. However, I haven't seen as much discussion about thinner differential oil. Has anyone done any experiments with this?

I wonder if something like GM Synchromesh or Volkswagen G52 could be used in my Miata's differential?

I don't drive the Miata hard at all...my favorite thing about the Miata is that it's incredibly fun to drive without driving fast or accelerating hard.

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Old 10-05-2011, 05:56 PM   #2 (permalink)
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Both my vehicles are rear wheel drive and the thinnest I would try is a synthetic 70w80 or 70w85.
Gear Lube - Oils, Fluids & Additives - Oils, Fluids & Sealer - SummitRacing.com
There is even a 75w60 weight, kind of different. Too bad it only come in 5 gallon buckets at a cost of $220.
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Old 10-05-2011, 06:14 PM   #3 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Both my vehicles are rear wheel drive and the thinnest I would try is a synthetic 70w80 or 70w85.
Gear Lube - Oils, Fluids & Additives - Oils, Fluids & Sealer - SummitRacing.com

Good idea! Is this an example of what you have in mind? Red Line Synthetic Oil - Gear Oil for Manual Transmissions - MTL 70W80 GL-4 Gear Oil According to the Red Line specs, this oil has 10.6 cSt viscosity at 100 degrees C.
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Old 10-05-2011, 06:27 PM   #4 (permalink)
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I would be much more comfortable using a 70w80 weight MT oil in a diff compared to something possibly way too thin like ATF or GM synchro oil.

If I ever decide to swap out my GM 10 bolt rear in my camaro for something else I will be sure to fill it with ATF and drive it for a while just to see what happens.
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Old 10-05-2011, 06:45 PM   #5 (permalink)
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I believe that the 65+ mods list does mention thinner diff oil as a possible way to reduce driveline losses. How thin you can go is definitely a very good question, one I don't know anything at all about. I've only ever owned cars with transaxles, where the transmission and diff share a single oil supply.

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Old 10-05-2011, 07:20 PM   #6 (permalink)
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yea i had the same question because im about to change mine and its never been changed so ive been told... (220,000 miles) I cant see it never being changed but i guess i will see when i actually change it. I just need to figure out what im going to change it to..

Think im just going to go with the 75w-90 synthetic
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Old 10-05-2011, 08:21 PM   #7 (permalink)
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I used to own a 1968 Volvo 144. I changed the differential oil in it to 75W90 synthetic. I don't know what I drained out, but it was definitely thicker than the 75W90 I put in.

Then the car developed a loud gear rattle at part throttle at 65-70 MPH, my normal long trip speed at the time. I believe that that rear end needed a slightly thicker oil. Any mileage improvement was very minimal, well under one MPG.

Hypoid gears need a minimum oil viscosity to prevent metal to metal contact.
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Old 10-05-2011, 08:34 PM   #8 (permalink)
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Depends on the gears really, and how the were setup, also how much backlash they have at time of oil change.

If mines starts to make noise i will go back to 80w-90 and consider it a flush of the old fluid
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Old 10-08-2011, 01:47 PM   #9 (permalink)
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Redline ger oil has an actual viscosity in the 50-60 range when hot. Works great even in high horsepower applications.
Check out bobistheoilguy forums for way better information than I can provide.
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Old 10-09-2011, 11:24 PM   #10 (permalink)
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One problem you are going to have is not so much the viscosity as it is the additive package. For a diff, you'll want a GL5 add-pack.

Syncromesh, Red Line MTL, Amsoil MTF and the like are GL4. I don't believe any of those are dual rated GL4/5 ... but some 75W-90s and 80W-90s are.

For really thin rear-end gear oil, I might try this stuff:

Red Line Synthetic Oil - Gear Oil for Differentials - 75W85 GL-5 Gear Oil

or:

Red Line Synthetic Oil - ShockProof® Gear Oils - Lightweight ShockProof®

The Superlight Shockproof is likely too thin for proper durability.

I think with a Miata, though, you're #1 enemy is wind resistance. I love my convertible, but it's an affront to the aero gods.

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