Sadly, I need to sell my 1970 Cutlass Convertible that I have had since HS graduation in 1985.

Working on a few maintenance items to get max dollar including a diff oil change.

I have some royal purple Max-Gear Synthetic Gear Lube Oil 75W90 left over. Can I use the syn stuff in this diff?

It is NOT a posi rear.

Do I need to add the LSD additive?

 

If it's GL-5 or better it should be fine. You do not need the additive. It is meant for posi.

 

From the Royal Purple website...

Max Gear is recommended for use in truck and automotive front or rear differentials, manual transmissions and lower gear units of marine engines that specify use of an API GL-5 or GL-4 fluid. It is non-corrosive to soft yellow metals (brass, bronze, copper, etc.) and synchronizer safe. Specially designed to extend gear and bearing life, Max Gear provides superior corrosion protection over competing conventional and synthetic gear oils.

 

For what it's worth, it's a Cutlass Supreme, not a Cutlass S.