I just got a Moser 12bolt with posi in it and am wondering what gear oil I have to use and if I need to use the GM limited slip additive or not?

 

Some fluids come with the GM limited slip added but check the bottle to make sure. Noticible gains have been achieved from using a synthetic fluid, so I would use that.

 

Both Moser and Eaton recommended not to use synthetic oil in the diff because of the carbon type clutches. I guess the synthetic is too slippery. I used 4 quarts of pennzoil 85-140W with a small bottle of Kendall posi additive.

 

Is 80W-90 ok to use?

 

80-90 is fine. And you don't "need" the posi additive. The Kevlar clutch facings don't need it. Some run it to quiet the chatter but I don't run any with my Eaton HD posi. I'd rather have it chatter a little on turns and hook harder on launch.