DSMZ28

iTrader: (12)

Hey
I will be installling Richmond 3.73 gears in my car soon (have them already) and I was reading the instruction book and it says to use 75w-140 syn gear oil.
My question is what do you guys run in the rear end of your vehicles. Because I'm running the GM synthetic 75w-90 now. Is the gm fluid fine or maybe even better then what they are recommending. Just curious. I have a lot of the GM gear oil at work. Also would i still add the limited slip additive if i went with the 75w-140 gear oil.Thanks

Adam

dailydriver

Since you have a '98, IF no one changed out your diff you have an Auburn in there. I would use a good mineral based gear oil (85W-140 summer, 80W-90 winter) and YES, you most definitely need the friction modifier additive with that diff!! Auburn and Eaton (the clutch pack ones at least) will void warranties if you use a synthetic gear oil in their diffs.

DSMZ28

iTrader: (12)

So I should not run the GM Synthetic 75w-90. My cars no longer under warranty but how would it void it if thats the gear oil GM tells you to use in it. Just curious. Thanks for the input.

DSMZ28

iTrader: (12)

Any others

dailydriver

I don't know what fluid GM tells you to run, but if you talk directly to the Auburn techs, they will tell you that you MUST run mineral based, NON-synthetic gear oil in their diffs. Since they actually manufactured the diff, I would tend to listen to them.

DSMZ28

iTrader: (12)

Ok so I'm installing the gears tomorrow with all new gm side and pinion bearings etc. Any more info on the gear oil that people run in the rear will be helpfull. Also do I need to run a heavier oil for a breakin period and then drain and refill later. Any info would be great.

Joe Vinci

iTrader: (3)

I run the standard GM gear lube and additive in my 98 Z. The C5s use the GM synthetic.