changing to a different differential fluid weight.
 

So my car manual says I should use 85w-90 gl4+ differential fluid in my front and rear differentials. I decided to switch to 75w-90 GL5 synthetic.

According to this I should be good on protection and pick up a 0.5% fuel economy gain.

https://www.fleetequipmentmag.com/sk...nner-gear-oil/

Any thoughts?

Well finally had a good mileage run with no red lights on the way to work. Got 28.5mpg which is good considering the sudden drop in temperature in the morning we just had. I will say for the first 5 miles my trip mpg climbs faster.

In summary I don't have a good baseline to compare things too with temps changing. The week prior to the change my best mpg on my work run was 28.3 . I will call this a win though.

Do it. It's now like you are putting 90w-140 in there.

I would say the mpg increase would be more than 0.5%.
Just changing the old thick and dirty oil and replacing it with the new gear oil in the same weight would be about a 0.5% increase.

Yea I would think it's more than 0.5% for normal street driving.

Yes, as the page says, there's nothing wrong with that. 75w90 just flows better when cold than 85w90 does. Both oils reach the same viscosity range when hot, so there's no damage being done.

Just don't go under the Xw90 range, unless you're doing it for a very short time to clean out a filthy differential, like I once did. The old oil had seen 180,000+ miles, 18 years, and it was filthy. I thought I should clean the filthy oil out, so I ran on two batches of 5w30 in a 75w140 differential, but I did it for only about 5 miles. Even then, I saw a few metal shavings in the oil, so cheap 10w40 would probably be better.

My only concern would be switching a German car from GL4+ to GL5. Try that with a manual transmission and the syncros will not work correctly. If your car has clutch positracks they may work differently.

The GL4+ recommendation is for GL4 or higher. The GL rating specifies the amount of extreme pressure rating additives are added. At least according to AmsOil. So going to GL5 shouldn't be an issue. Also the diff fluid I used is on the Mercedes approved list.

https://blog.amsoil.com/the-differen...gl-5-gear-oil/

As long as Mercedes approves you should good! Air cooled VW gearboxes did not like GL5, it is too slippery an oil for the syncros.

What if there was a VERY easy way to ceramic coat the diff's wearing and bearing surfaces and the ceramic coat was 80X slipperier than steel-oil-steel and 85% the hardness of diamond..??

Sounds too good to be true right? :)
Snooping around Argonne National Labs' web site will do 2 things:

• Allow you to do exactly that.
• Make it so you actually believe the claim..! :)

I have:

• Restored, smokey, worn old engines to health, with a noticeable improvement in fuel economy or power, depending on how heavy your foot is. (Ceramic Layer is 0.5 micron thick. Thats a total of -2 microns of play in a bearing or piston/cylinder)
• 'Buggered up' a gearbox as the syncros (that rely on friction) stopped working to the point that required the 'gear changing without a clutch' trick.
• Restored whiney old diffs. (non Limited slip! Unless you want to unlimit it!)