Hi,

I was thinking about going with the Redline synthetic gear oil 75W90 with the friction modifier. However, I have heard/read people suggest to use the regular 80W90 gear oil instead in a 8 3/4 with sure grip.

I think both the Redline (depends which one) and Lucas differential oils contain the limited slip additive already. So I guess this is more of a synthetic vs conventional question than anything else.

The car is a 1968 Barracuda with a sure-grip unit, 3.91's, street driven, not drag raced. Differential was taken apart to inspect and now going back together. Everything looks really good (bearings, gears) so no new bearings or gears. They both have about 15-20k tops on them.

What have you used and/or currently using that has worked for you? What would you recommend for my application?

Thank you in advance

I would go with any good synthetic that is approved for limited slip differentials. I don't know if you ever have it out in the Colorado cold but I think it would be better for that.

I know that my race car would always run a tenth slower on the first run of the day until we switched to a synthetic rear end oil. After that the first run was as good as the second. I have seen others that have had the same experience.

However I run a spool and don't have to worry about taking car of the clutches etc.

Mobil 1. But only because I have tons of it, and it was free.

If you have a power lock, you will need some more additive
to allow the clutches to slip. If you have cone type, you can probably get away with just the oil.

Good choice and no additional friction modifier needed.