Guys, I have the BW 9-bolt in the ride, it has posi.

I want to go with synthetic gear lube. The Royal Purple stuff says that the posi additive is NOT needed. What about the Mobil 1 synthetic? Does it need the additive? Or the Redline?

If the synth oils have the posi additive built-in, I don't want to add more, then it would be too slippery and I would lose the posi action altogether.

After all, isn't GM now selling just a full synth for these rear ends, and no additive???

Thanks!!!!

 

I also wanted to run the Royal Purple or Mobil 1 gear oil, but couldn't get a "for sure" answer on the friction modifier. I went to the dealer and bought it from them. It is a full synthetic, but it has a "built-in" modifier for the posi. After talking to my hook-up at the dealer, there is a difference and he said that he's seen them burn up with out the right stuff. He said that he's only seen a couple of the 9-bolts break, but in all those cases, except one, it had the wrong oil in it. The one that didn't have the wrong oil in it, didn't have any oil in it.

 

The redline oil I bought already has the friction modifier in it. I believe it says so right on the website.

 

OK, it sounds like both RP and Redline specifically state on their websites that the friction modifier is already in the oil, so use of an additional modifier is not needed. That leaves M1 as the only unknown. I've sent them email, I'll let ya know what they say in response.

 

They do take 75W-90 right.

 

Red Line doesn't state not to use the (GM) posi additive.
I have info from Red Line, where they only say one can use LESS additive!
I'm running Castrol full synthetic 75W140 axle fluid with one bottle GM additive in my 9-bolt!

 

So does it call for the 140 i cant find what it calls for.

 

75W90 is allright, 75W140 is only SLIGHTLY thicker when warm, imo better for an axle with 100000+ mls!

 

OK, I mixed one quart of Redline 75W-90 with one quart of Royal Purple 75W-90 (both full synthetics with "built-in" LS additive) and refilled my 9-bolt. I did NOT add any additional additive. So far so good, no posi chatter or klunking noises. OTOH, the posi in my 9-bolt is well worn. Let's see what happens with time and miles.....

 

so does any one have any ideal if you the Mobil-1 synthetic has the friction modifier already in the oil?