Black RTV is commonly used in place of the gasket on rears. Your choice. If you use the gasket + RTV do not torque the bolts until the RTV is set up.. a couple of hours. Otherwise the gasket may split and squeeze out. Either way, clean well first (razor or putty knife + lacquer thinner or brakeclean).

For the lube... with general use I believe any 80W90 lube will work fine. If the rear is limited slip, a tube of additive is needed. Two tubes? Maybe for a big PU rear but suggest one tube first and add another if it chatters.

If you are towing hard, heavy and fast, 75W140 synthetic will provide more protection for high temp situations.