The shop has started rebuilding my rear end. It's a 2002 1500 Surburban with limited slip differential. They are going to add the GM additive (GM PN 1052358) before I pick up the rear end. Their salesman said that with this additive, I didn't need to use 75W-90 synthetic oil, but could use any name brand 75W-90 oil which complied with GL5 specs. GM recommends synthetic oil in the owner's manual, but makes no mention of the additive. If I have the additive, do I need to use synthetic gear lube? Thanks.