First thanks to all those who made suggestions and the fella that gave me actual GM ref. numbers and cross ref. numbers when last week I was trying to figure out what kind of shocks to buy for my 88 olds cutlass supreme classic brougham that has the F41 susp in it. Got shocks done now I am trying to figure out differential oil s for this car. I am having no luck with internet search so I am back turning to the pro's on here.
Its got a 3.08 rear end but do i go with something with additives or not or dont matter? and a 75/90? Conventional oil or sythn for diff. fluid ?? thanks in advance... Rj

 

Buy conventional 80-90. Does it have a LSD in it? Some lubes now come with posi additive in it.

 

You lucky Ducky, someone checked off the best options possible on that car. Neither the F-41 or optional 3.08 gears were checked off on my 88 CSC Brougham that I had. If open, either mineral 85W90 or synthetic 75W90 will work fine. Almost no posi recommends synthetic fluid FYI. Most fluid has a friction modifier added, they will say approved for limited slip differentials. I used full synthetic 75W90 and 75W140 with the proper additive with a Trac Lok and Gov Lock in my trucks that see -40 without issue.

 

No, it does not have posi

 

I use Royal Purple full synthetic 75w-90 in my 8.5" rear - also 3.08 gear but Posi as well, so I also added a separate Posi additive in further precaution.

Good stuff and readily available at any decent auto parts store.

 

Does it have a tag on of the diff cover bolts indicating limited slip?

I realize your asking about the diff so there aren't any yellow metals...Recently had to source (special ordered) GL-4 for a '60 manual transmission to avoid GL-5 and what it can do to synchronizers (yellow metals). Many gear lubes said GL-4 and 5 compatible but I wasn't chancing it on this car.