Just finished rebuilding my diff the other day and just wondering if anyone is using synthetic gear oil or just using what the book says with the posi additive?

Also it says to drive her for 10 miles and let the gears cool for 30 minutes .. which I have done but then it says to change out the oil in 500 miles.....

Pete

A tranny shop said always run the posi additive.

This can be an interesting debate. I've followed a similar one a few times on other forums. I'll put my 2 cents in on what I've been taught. Regular GM posi oil, no synthetic, no posi additive. The argument I've heard against the synthetic as well as the additive is they provide too much lubrication effectively eliminating the holding power the posi clutches have. I am no expert but it makes sense to me. Supposedly the additive is to add to posi's that chatter to reduce customer complaints. It's all I've ever run in many, many posi's and never had trouble with one.

If it's a performance car and you want the posi to lock hard don't use the additive.... I can live with a little chatter from the clutches... I can't live with a posi that doesn't grab. I used the additive once.... never again.

I have always added one small bottle of the GM additive to mine and I still get chatter when I gun it going around a corner; it hasn't affected the operation of the posi at all.  Without the one bottle added, I can't imagine how bad the chatter would be.