69 Cutlass, factory Chevy 12 bolt rear. Not a limited slip. Chassis manual calls for sae 90, hard to find.
Can I safely add a multi viscosity gear oil?
Thanks

 

Yes.

I know this is for the rear diff but try to avoid GL 5 if it is a 4 speed car because you can't multipurpose the fluid. GL 5 likes yellow metals in manual transmissions i.e., it eats them!

 

You can use 80/90 or 75/140 if you're not driving in extreme cold winters.

 

I do have some 80-90 conventional on my shelf and will top off with it. The straight 90 is hard to find and expensive.
thanks

 

I do not know many places that sell straight weight oil anymore.
Any rear diff fluid will be fine and provide good protection
Open diffs are really tolerable to anything.
I always tell everyone...LOW or NO fluid is what will really mess a rear end up. Lubrication is KEY ...keep the level up of WHATEVER you install. Jim