All I can find is 75W-90? Is that the recommended? What's the differential's fluid capacity?

JamesC

 

Yes that would be the stuff.

The rear holds around 2 quarts, depending on which cover you have, which carrier, etc.

If you have a clutch type posi such as Eaton or Auburn, add a bottle of the GM additive first, then fill it the rest of the way with the M1.

 

Word of advice:

Read your diriection for the cone clutch type posi's- You are not to use synthetic type gear oil with them.

 

I had Autozone look up the capacity for mine, it took 1 3/4 quarts. I'm sure you could have them look it up if you stopped in.

vsixtoy,
I dont know much about posi units, can you tell me what a cone clutch type is? I've got a newer ('97) rearend, 3.23 ratio with posi. I just added Mobil 1 Synthetic, and I hope this was ok...
If not, !

 

I believe a '97 rear posi unit is a Zexel?

Don't think they are a problem- I never owned one though. Deffinately don't use synthetic on an Eaton or an Auburn.

 

Caution:
I used Mobil 1 gear lube on my high mileage Auburn Posi , I added the slip additive. The rear was making loud noises and clicks right after the lube change, I thought that the posi broke. I replaced the Mobile 1 with regular gear oil and additive, the noise dissapeared in 10 minutes. The Mobile 1 was not thick enough, I wasted $35 (gaskets, oil, etc) for nothing. Just my experience
Enjoy

 

I ran Mobil1 in my old 10 bolt with an Auburn for several years with no problems. I'm currently using it in my 9 bolt and it's doing fine as well.

 

For all the information you would ever want to know about oils, gear lubes, filters, even oil for your lawn mower go here. Some of the info there is truly eye opening.

I'll never think of oil as just plain old oil again after visiting this site and reading up on oils and filters.

http://theoildrop.server101.com/ubb/ultimatebb.php

 

Synthetic's fine for an Eaton. Have been running it for years. Amsoil and Redline. Zero problems. You may have to add GM friction modifier. I don't.

Particularily if you Autocross or Road Race, Synthetic is a necessity.

All of the top Autocrossers out here run synthetic in their Auburns as well. With a 10 bolt Auburn the only problem with a synthetic is that it is too slippery and you lose some posi affect. That can be cured with different Posi load springs. With 9 bolts it's a no-brainer. We just shim the livin' beejeezus out of the things ( 150lbs to 300lbs break away torque ).

Autocrossing is a hell of a lot harder on Auburns than Drag Racing. If you Drag race, I'd just throw in the Synthetic oil ( 75\90w or 80\140w ) and not worry about things.

Torsens must absolutely run Synthetic. No if's, and's or but's.

 

I'll throw in a data point on this...I'm running a Torsen HD with Redline Heavy Shockproof synth gear lube, and it's a happy camper.

Next to zero metal on the magnets when I changed it after a hard season on the road courses, and very quiet even with noise-prone Richmond gears.

 

I also run synthetic with GM additive with my Eaton. no problems, no chatter.

 

In the owner's manual for my 2002 WS6 it calls for synthetic rear end lube. My Auburn unit that I put in my '88 said not to use synthetic.

 

Exactly

So a note to all that use it- synthetic may work for you but the manufacturer specifically says "don't use it". Thats all I'm saying. I don't think there is more than a small handful here that can properly set their own diff pressures to compensate for slippage of the clutches with synthetics as not to burn them up.

 

I have Mobil 1 with GM additive in 2 posi rears I own, and have for many years now; one is a stock Auburn, the other is an Eaton heavy-duty. No problems here.

 

Here is the catch on the "don't use synthetic" issue. I went to the dealer to get the correct lube for my 9-bolt. I took the manual in, and got the # it called for. Low and behold that stuff has been discontinued, and replaced with... synthetic. It is like gear lube + friction modifier all in one. I asked if he was sure that is what I needed, just to be sure, so he showed me the info on the computer. Sure as hell, it is synthetic, and sure as hell it replaced the lube for the 9-bolt. I think that I'll trust GM on what to run in my diff. The funniest thing about it is, it was about the same price as Royal Purple Mobil 1 etc.