I posted this in the drivetrain forum, but I thought I would run it by you guys too...

Has anyone used Royal Purple 75W90 gear oil for their rear diff? I debating on using this synthetic oil this time or going back to the regular dino oil. I've got cheapy Crappy Tire synthetic in there now...and the additive. But if I go with Royal Purple, I won't be using the additive this time.

 

I didn't notice much of a difference b/w the royal purple and the regular stuff I buy from GM... I think though if I ever get the shims right (I guessed lmao not exactly the brightest thing to do, but 4 years later and still works) I think I'd probably be able to hear a definite difference as royal purple I've heard really is good for quieting down the gears if they whine...

 

Thanks. So you tried both with no noticeable difference? Hmmm, and the famous question, did you or didn't you us the additive?

 

Being that my diff is open, I don't have to use an additive. As I recall there was some mention of royal purple not requiring the additive or something about synthetics not needing the additive for posi not sure which case is true... As long as the oil is certified for use in posi without an additive, I would not add an additive as it was taken into account in the formula for the synthetic. Adding it would only cause problems I recall.

 

I doubt it at least from my experience... I used 75W90 Synthetic Gear oil for my rear diff and I threw a bottle of the GM Limited Slip additive when I changed the gear oil in my 3.27 posi.

I haven't seen any ill effect of doing it...actually it made an improvement... here is why I say that...

Before gear oil change I had...

1. Periodic rumbling from the rear when taking hard, sweeping right turns ,

2. 1 and 1/2 wheel peels :-) passenger side would smoke and the drivers side would try to spin.

After I changed the gear oil with the CT 75W90 Synthetic Oil and added the GM Additive..

1. No more rumble was so ever. I can pull as hard a turn as I want... rear stays quiet

2. Two wheel peels... leaves two nice black stripes (at least she did the last time I power braked her up to 1100 rpms and stepped on her!! hehehe

Is my posi on it's way out? Has my rear diff seen better days? Yes and Yes...

But for now I am going to be using this posi addititive and synthetic oil until I get the cash for the following:

12 bolt Moser Posi 3.42 ... but that will have to wait for several more years.

 

If I do go with the Royal Purple stuff, it does specifically say on the not to put in any additives, so I won't. The CT synth didn't say anything to that effect so I did add it. But after a year and half, I'm wondering why it appears to be causing a noise. (At least I'm hoping that's the noise that I'm hearing.) Hence why I'm changing it again.

 

FYI I put in additive in my 9 bolt and sometimes it only lays on one..Maybe I put in too much

Daz

 

Possible...I've talked to a couple people who have only put in a half or 3/4 bottle.