Here's what the engineers at Eaton recommend for our old posi (LS) carriers. From here (scroll down to 'Eaton Posi'):

http://www.eaton.com/EatonCom/Produc.../FAQ/CT_128414

...is this:

"What kind of oil should I use? Can I use synthetics? Do I need a friction additive/modifier?
Eaton Posi units perform best when using GL4 (or better) mineral/petroleum based gear oil. A four ounce bottle of friction additive/modifier is also necessary for optimum performance."

 

The friction modifier was added at the factory when they originally filled the diffs. It is in there for a reason, because the fluids themselves didn't have the right properties needed for the posi unit.

I went to a local oil dealer and got a gallon of diff fluid with the modifier already added in. I still have over half a gallon left so I know if I ever need to change it again, I have the right stuff.

 

GL5 already has the friction modifyer in it and no additional additive is needed.

 

This issue has been kicked to hell about adding the GM additive endlessly! I think that the answer is pretty simple-conventional oil, use the GM additive. Full synthetic GL-5, not needed or required. The Eaton recommendation was made when the dinosaurs were still living and their 50 year old recommendation does not apply to newer lubricants (synthetics) which are far superior to conventional oils-proven time again. I switched to Mobil 1 75W-90 with no additive in 1990 which would be 21 years ago now, have had the diff cover off, inspected the gears, and they looked brand spanking new. The differential has never chattered/shuttered once since I add the synthetic diff oil with no additive and I have seen the gears after running the synthetic oil with no additive. If you feel better with the GM additive, then by all means, add it but it is not necessary with today's synthetics.

 

I am with JB on this one..I like the synthetic idea..probably cheaper since the GM bottle is somewhere around 10$..Done deal.

 

Are you saying to your fellow C3 brethren that running a synthetic is ok?

The Eaton recommendation:

http://www.eaton.com/EatonCom/Produc.../FAQ/CT_128414

...is still up today. Also, if you purchased a new Eaton limited slip carrier, today, it would come with a big green tag attached with a thin wire that says, "This Limited Slip carrier is designed for use with mineral/petroleum based gear oil, GL4 or better. Please see the enclosed recomendation for friction additive (if Chevy there's an enclosed GM number and if Ford, the Ford part number). Do not use synthetic gear oil".

Also, isn't in interesting that Eaton recommends against the use of synthetic gear oil with the use of their newest designs, the Detroit Locker and the True Trac (see the Eaton link above).

Still not convinced?

Call the technical desks at these companies:

Strange Engineering,..847-663-1701

Moser Engineering,..260-726-6689

Tom's Differentials,..208-265-8111

All three are big Eaton Dealers and giants in the industry. Ask them what they recommend for gear oil for the Eaton Posi (LS) carriers they sell and install in their differentials. Ask them if it's ok to run a synthetic gear oil.

Still not convinced?

Did you know that Auburn and Dana/Spicer also recommend against the use of synthetic gear oil with their new LS carriers.

Run what you want, but just because a product is newer, more advanced and more expensive doesn't mean it's good for old rigs. Dot-5 and Dex-Cool come to mind.

 

None of the responses on that web page say "Do not use ...." - they say "We do not recommend synthetic oil."

Nowhere does it say why.

It does say (repeatedly) "...GL4 (or better)" - and my understanding is that GL5 qualifies as "..better".

Many of us have been using Synthetic for years with no problems and quieter gears.

 

"What kind of oil should I use? Can I use synthetics? Do I need a friction additive/modifier?
Eaton Posi units perform best when using GL4 (or better) mineral/petroleum based gear oil. A four ounce bottle of friction additive/modifier is also necessary for optimum performance."

How can the above, from the engineers at Eaton, possibly be interpreted to mean that running a synthetic is preferred, recommended, or even ok. "Performs best"?,.."optimum performance"?

Again, the 'green tag' attached directly to a brand new LS carrier from Eaton specifically says, "Do not use synthetic gear oil". Is that somehow ambiguous?

And again, did you call Strange Engineering, Moser Engineering, or Tom's Differentials (all Eaton dealers who build performance differentials) to see if running a synthetic gear oil was ok with the Eaton's LS carrier? Their phone numbers are posted above.

Why the recommendation? According to Eaton, the lubricity of synthetic oils is far superior to mineral/petro oils. The LS clutches don't perform as designed and limited-slip funtionality can be reduced and/or clutches become 'grabby'. They'll tell you there are too many variables with the use of synthetic gear oils.

As a lab test, one could raise one rear wheel, try to turn it (with the other on the ground, tranny in neutral). There would be a difference in effort needed to turn the wheel depending on if the differential was filled with a synthetic or petro gear oil. Right?

Again, if you run a synthetic gear oil, one of three things will happen:

1) It will run great
2) It will bump and chatter
3) Limited Slip (positraction) effectiveness will be reduced

If you run the recommended mineral/petroleum gear oil and the 4 oz. bottle of GM additive, one of one thing will happen:

1) It will run great

 

I normally use most major brand 80W90EP gear lubricant with no other additive's and work's fine. Been awhile, so not sure this stuff is commonly availible at most auto part's places. Pretty sure lot's of boater's/marina's are still using it though.

 

What about Amsoil 75w 140? Its for extreme conditions and I happen to have 5 quarts on the shelf. http://www.amsoil.com/storefront/svo.aspx