Crownman

Yes, the oem LS, or T-lok needs friction modifier. Some gear lubes already have the modifier in it and some don't. It will say on the bottle.

qdeezie

85w-140 or 75w-140. You don't need any friction modifier since you have an open diff.

choate

I'm going to use valvoline 80w90 with the friction modifier already in it. It's like 9 bucks a bottle

Edit: the full synthetic is only like 2 dollars more a bottle. I would go that route since it's not much more