90wt Hypoid Gear Oil is the original lube for the rear end and the manual transmissions. If you have Posi, you also need to add the posi lube.
There's a plug on the side that you fill it from, and when it starts leaking out, it's full. As for exact quantity, I "believe" it's a few quarts, but I'm not close to my service manual right now to verify that. I'm sure someone else can...