Posi oil question

[Tremo]

Yo guys, I'm looking at changing the oil in the rear end again, I want to do it up right. The car has a Borg-Warner 9-bolt diff with positraction. The factory calls for a 80W-90 GL5 oil with GM posi additive.

I want to go with synthetic gear oil this time. I was noticing that the Royal Purple oil claims that you do NOT need to use posi additives with their gear oil. What about M1 and Redline?

I don't want to add posi additive if not needed, or add too much, because then it gets too slippery and I lose the posi action altogether.

Any insight is appreciated, thanks.

 

[7TFord]

Posi recommended lubricants come with a certain amount of friction modifier in them but the question is whether it is enough or not. My restored 70 Mach 1 has a locking differential and after I had it rebuilt I had to add quite a bit of extra friction modifier to get it to stop chattering while cornering. Your diff will likely function properly with the amount of modifier that is present in the lubricant but you will have to change out the fluid and try it. I would opt for the Redline product personally.

 

[Tremo]

I've heard stories of guys having to add 2 or more bottles of friction modifier to stop the posi chatter, in some instances.

Way back when my old Buick has posi chatter but I had no idea what it was at the time. Oh well....

I just want to do the current ride up right with the good stuff, but not "over lube" the posi so it stops working.

I'll probably give the RP or Redline stuff a shot and see what happens. If it starts to chatter, I can always dump in some additive. Regardless, if anyone here has direct experience, I'd sure like to hear it.