I drained my 12 bolt to change the pinion seal and do some painting. A rearend shop told me to refill with 90 wt Valvoline gear oil and not worry about the posi additive unless it chatters due to the age of the clutches. I believe that this rearend has the original cluthes and seems to work ok. Does this sound like good advice? Should I go with original GM oil? Why the big difference in weights of oils that are suggested to be used? Thanks for an "education".
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Gary

Gary;

You're in the hotter central valley area - I would maybe recommend 140w over the 90w. That's what I run in all of mine.
I would always add the 'posi goop' by GM, specially to an older set-up, to assure the clutches do as they are designed to. If you wait until they become glazed or damaged and then add the 'goop' - what sense is there in that :confused:

I hope BGH, Chicane and others chime in too;

John

If this is just a street car then I would go ahead and ad the additive. If you race it then I would leave it out unless you have a chattering problem. I use Amsoil synthetic oil in the rear ends that I build. The 75W90 Series 2000 and 75W140 Series 2000 are made for limited slip differentials (posi).