Eaton Posi and lubrication update - Team Camaro Tech
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post #1 of 3 (permalink) Old Oct 16th, 15, 01:49 PM Thread Starter
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Join Date: Jun 2001
Location: Central Minnesota
Posts: 15,851
Eaton Posi and lubrication update

Over the years, many of us have replaced old differential gears with new units. One of the most popular is the Eaton Posi gear sets. In the past, the box came with a label that scared people away from using synthetic lubes in those new gears. While the lube makers gave warranty on their lubes in Eaton gears, folks were scared away from synthetic lubes anyway. Eaton claimed they didnt have the time and resources to test every gear with every lube so they just recommended the old fashioned dino lubes.
Today is different. I was sent a new Eaton Diff. owners manual and installation guide. In it, Eaton has now admitted that synthetic lubes are better in their units than petro lubes and they now recommend them. Here is their quote direct from their manual:

Eaton Posi™
Quality mineral or synthetic lube with sufficient friction
modifier content will ensure the smoothest operating
and longest life of your Eaton Posi performance
differential. Eaton recommends GM Dexron LS or
Castrol Carbon Mod SAF (Synthetic Axle Fluid) for all
Posi applications. If using non-“LS” gear lube, add four
ounces of General Motors “Limited Slip Axle Lubricant
Additive” per one-and-a-half quarts of gear lube (GM
P/N 88900330/10-4003). Additional friction modifier
additive may be used to reduce LSD plate “chatter,” but
do not exceed eight ounces per one-and-a-half quarts
of lube. Other brands of additives may be used. A
friction modifier's purpose is to lower the static
coefficient of friction (grabbing) within the clutch to
match the dynamic friction (sliding) in an effort to
reduce the effects of friction transition (stick, slip,
chatter). Excessive amounts of a friction modifier can
lower both friction values, resulting in reduction of bias
ratio (performance) for plate-style limited-slip
differentials such as the Eaton Posi. Many brand name
gear lubes contain some limited-slip friction modifier in
their formulation. If you use such a lube, additional
friction modifier additives should be used sparingly to
avoid accidental over-saturation resulting in
performance loss.

In a nut shell, anyone who was once swayed away from synthetic gear lube in differentials, rest in knowing they are perfectly fine to use and in fact they work better than petroleum gearlubes in all applications. In fact, alot of new trucks come from the factory with synthetic gear lube in them already.
I thought this might be of interest to newcomers who hear old wives tales from the past. Jim..former owner of a 69RS-LM1 350/255 - 700r
"There will come a time when you believe everything is finished,
That will be the beginning." Louis L'Amour

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post #2 of 3 (permalink) Old Oct 16th, 15, 06:53 PM
Senior Tech
Join Date: Feb 2000
Location: Central City, Ky.
Posts: 5,587
Re: Eaton Posi and lubrication update

GM trucks with the Eaton Governor Lock have had synthetic oil in them since '99. The Eaton Gov Lock uses the same clutch discs material as the Eaton Posi. That was proof enough for me. I used Amsoil with the Eaton Posi in more than a hundred rear ends with no problems.

'69 RS/SS396 pro street/427/4spd/9"

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post #3 of 3 (permalink) Old Mar 28th, 16, 08:16 PM
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Tom Anderson
Join Date: Oct 2014
Location: Southern California
Posts: 267
Re: Eaton Posi and lubrication update

I just bought a 12 bolt posi and had it checked and was told it is in great shape . What type of oil is best to use and how much of the posi additive do I use ? I was told it will hold 2 1/2 quarts
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