What type oil to use?

[Blackbandit]

Hey guys, just picked up a gasket and the oil for my rearend but I have a problem...I don't have an owner's manual with the car...I downloaded one but not sure if it is right for my car..

I hear alot of people using 75w 90 synthetic but the guy at the parts counter at GM gave me 80w 90 non synthetic and said that the additive was already in the oil.

In the manual I am reading, it says that if I have a limited slip differential then I should use the 75w 90 synthetic....I don't know what I got.

I do however have the rpo code G80 on my door which says posi rearend.

Can anyone help me out on what to use and the additive? Thanks..Steve

[Cutlass]

75w 90 synthetic and I like to throw in the LSD additive for good measure. Some say you don't need the LSD additive.