I'm an "expert"
on G80s so just to clear up some common misperceptions about recommended gear oil here's the skinny.
The Bulletin that Jamison posted is one source of confusion and in particular this part ...
The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.From the above it seems as though friction modifiers are not recommended for use in G80s but that's not quite right.
Automatic Locking Differential Lubricants. [Note from Michael Asmussen, Torque Control Products Division of Eaton Corp.] We recommend the following lubrications for our locking differentials:
1)Texaco 2276; Synthetic 75 W90; Gm Part # 9986115
2)Texaco 9622; Mineral based 80W90; GM Part # 9985290
3)Texaco 2080; Synthetic 75W140 (heavy duty applications); GM part # 9985991
Note - All of the above lubes are preblended with friction modifier. No additional modifiers are necessary or recommended. As far as other lubes are concerned, any standard GL 5 lube will work, but the units perform optimally with the three listed above.All common gear oils have FMs which is acceptable provided they don't contain a high concentration and they do vary. I called Valvoline and their Performance dino gear oil contains 3% FM. Other brands can have as high as 8% FM.
Here's another issue concerning the use of synthetic gear oil from Amsoil.
[5] Do not use synthetic lubricant in axles using RTV sealant.
American Axle has only approved the use of Synthetic GM P/N
12378261 as follows: All 8.6" rear axles.All 9.5" rear axles
built before mid-FEB 1998 are not compatible, after mid-FEB
1998 are compatible.All 10.5 inch rear axles built prior to
March 3, 1999 are compatible if axle shaft hub RTV sealer
is replaced with GM gasket P/N 327739.Hope this helps anyone that wants to service their dif.
