is it ok to run synthetic gear oil in the positrac/performance rear end?...for example royal purple 75w90....or will it leak

 

Leaking isn't the issue. Eaton and Auburn both say do not use synthetic fluid with their carriers because their friction discs aren't compatible with it. I believe factory posi units were made by Auburn.

 

thanks for the info....i will stay with the regular

 

five7, has your opinion changed about using synthetic in the diff? I 'seem' to recall that you were a proponent at one point. There are members, including me, who use Mobil 1 75-90 with GM additive and whose cars are equipped with an Auburn. No issues for me after a number of years.

JamesC

 

I have a question. What weight would you reccomend for general use on a stock 9 Bolt with 3.70's in it with a TA diff cover? Anything different than normal? Any addative? GM posi or Ford Friction Modifier?

If there is a FAQ about rearends i would love to read it, just point me in the right direction.

Thanks

Keo

 

Closest thing to a FAQ for 9-bolts:

https://www.thirdgen.org/forums/faq-...now-about.html

JamesC

 

Go for sticky!

Thanks James

 

(Sorry, didn't see the follow-up responses.)

I am a huge proponent of synthetic lubes. I picked up a used factory posi unit, worked fine for about a month after install and switching over to synthetic lube, then turned into a one-wheel-peel unit.

I had a new Auburn unit put in the '57. I called AMSOIL to get their take. They said exactly what you said (except about AMSOIL, not Mobil 1, of course). It failed to hold after about a year.

The instruction sheet that came with the new Eaton unit I installed in the Camaro last fall was very specific about not using synthetic gear lube. It held fine through the fall racing season, and winter driving, still good and tight today with petroleum-based gear lube.

I wish they'd get their act together and use friction materials that are compatible with synthetic gear lube. Until then, I'm going to go with a locker-type carrier such as a Detroit whenever possible (didn't have time to get one when doing the diff last fall, wasn't one available locally).

 

five7kid,
what is the milage span difference between the synthetic and normal gear oil? I guess what im asking is how long is each one good for?