I searched for over an hour at work yesterday, and it seems everyone is ALL over the place. My 9 bolt has 72,000 miles on it. I have NO idea when it was changed last. When I searched I came up with these results:

"Use synthetic and the GM posi additive"

"Synthetic acutally made my POSI not work, switching back to non synthetic made it work perfect"

"My POSI was fine until I added the POSI additive, so I just used the oil and no additive and it worked great"

"GM says do NOT use a POSI additive in the 9 bolts"

"Using redline fluid with no additive worked awesome, but redline said it is only for intense high performance applications and my cones are going to burn up"


Anyone have any solid info on this? I didn't think I'd have so many decisions changing my gear oil, lol.

 

well I can tell you this, I have been told to ALWAYS use an additive if you want your posi to last, I have the same rear you have and I use additive in it, as the matter of fact it`s the regular ol` stuff you can get at advanced auto, and my rear is fine and strong, frankly, it is even quiter and smoother when your doin slow tight turns than it used to be!

 

no additive added when I ran mine for a few years beating on it. Just swapped out the fluid in a buddies at his shop and no issues either with no additive just regular old fluid nothing fancy.

 

I change my 9-bolt's fluid at 85,000 miles, cleaned the case out with a can of brake cleaner let it air dry overnight and added GM fluid and GM additive. After a test drive it didn't feel like it was the same. After all the reading I might drain and refill with straight fluid to see what happens!! Tim.

 

Just used redline syn. Little under 2 qts. Supposedly has the additive already in it. Has been working great for over a year-very quiet and no chatter!

 

The GM additive is for clutch-type posi's like the 10-bolt. The 9-bolt uses cone-type posi's and no additive is needed, but it will not hurt it.

You can use regular 85w-90 gear oil, or synthetic rear end fluid.

 

Changed the fluid in my 9 bolt, ran with no additive, made lots of noise.
Put in the additive and it runs smooth and quiet.