Hav a 1988 Trans Am GTA with Auto Tranny 5.7 350 TPI Engine I do not know wat kind of rear end I have??? Can anybody help on how to find out??? Is it 75W-90 or 80W-90 or?? If it calls for 80W-90 can 75W-90 be used with no problems or loss of performance to ur gears??

 

GTAs came with the Aussie M78 9-bolt rear end.

85w-90, IIRC. But I wouldn't get TOO hung up on the specific of the oil weight.

Check out http://www.gtasourcepage.com for any GTA specifics you wanna know.

 

Either of those two weights should be fine. I'd avoid using a full synthetic though. I've read posts on here from other 9 bolt owners that said the full synthetic oil causes the 9 bolt posi unit to act funny (i.e. jerking or chattering). I've actually seen 140 wt. gear lube before, and that's a definite no-no since it's too thick to climb around and lubricate everything properly.