If i were you, to be on the safe side, i would use any synthetic 75w-90 or 80w-90 gear oil and GM limited slip additive. (something like castrol, valvoline, etc and the additive you get at the dealer).
im saying this cause with the 9 inch you cant just pull the diff cover to see what kinda limited slip is in there, a gear type or one that uses clutches like an auburn. probably you have a locker type posi in there like a Tru Trac. Since u can use synthetic diff oil in pretty much any diff if your unsure of whats in there you should be fine. but DEFINITLY use the friction modifier just in case you have a limited slip that uses clutches not gears.
99 Z28 M6
Blk/Gry lea. SS clone, SS hood, 10 spoke 17's, R.S. gear built Tranzilla T-56, Hays clutch, SLP Flopac CAI, SLP Loudmouth, Bmr LCA's, UMI APHR, Anzo Halo's, Hurst Billit Short throw.
79 Z28 355/Th400
"1/4 mile at a time, thats just fine"
nitrous till it pukes