All,

I am about to tackle my leaky pinion seal this weekend and I had a question around friction additive and gear oil. I purchased the Ford additive from Dr. Diff and I have a stash of Valvoline synthetic gear oil (75-90). The bottles say it already contains the additive for limited slip diffs.

My 8-3/4" rear has an Auburn cone type unit...should I add the Ford additive or not? Maybe use half? Is there such as thing as too much additive where it could have a determinantal affect?

Any insight is much appreciated!

I wouldn’t add any to start......because....
1) your fluid already has modifiers in it.
2) cone type differentials do not seem to be as “demanding” about additive as clutch type
3) it’s easier to add it than take it out.....if your diff experiences chatter or other issues then add it later

Good point on #3. I will l will leave the modifier out and see if it gets noisy.

Noisy? I don't think noise is what you're looking for. Go to a parking lot, drive in a figure eight. If you need some modifier, you'll know quickly. I think I endded up with about a 1/2 bottle with my full synthetic gear oil, but it had no modifier in it.