Gear
oil is a lot like motor
oil. If you're beating on it pretty hard, you should use something heavy, like 85W-140. But if you're using it lightly, a lighter
oil will work better, like 75W-90. It takes about 3 pints. Get two 1 qt bottles and you'll have plenty for the diff, and some left over to pour on your shirt while you're laying under there.
If you have limited slip, don't forget the additive.