I have a question about what fluid to use in the rear axle. The owners manual says 12378261 is the GM part number for the fluid. This has been replaced by PN 89021677. According to the manual, there is no difference for locking rear ends.

Most GM cars owner's manual says you need limited slip additive (PN 1052358) if your car has a limited slip axle. They only list the old fluid PN.

I have heard some different things on the Cadillac forum. The new stuff smells like grapes. My dealer and Cadillac said the new PN does NOT need the additive. However, a bunch of people on that site say it does.

There are a lot of different non GM gear oils that supposedly work but doesn't meet GM Spec 9986115. I have no idea what it means and I can't fiind it by searching the web. Anyone know?

Has anyone changed their rear end fluid on their Trailblazer? I had the Chevy dealer do mine and they used the new stuff (at $26 a quart). I don't have a locking rear in my Chevy though.

I also question GMs spec number. They can easily tell if you don't use their fluid because it smells like grapes. No other manufacturer fluid said it meets that spec. No one will admit their fluid meets that spec also.

I'm changing the rear end fluid on my CTS V and I just want to do the right thing. I'll probably go with the GM spec stuff and suck up the $30 extra cost for 2 qts. I will have the dealer do it. That way if they don't put the additive in and something happens, I'm covered. If I put it in and something happens I may be screwed if it is not required.

Sorry for the long rant but if you change your rear end fluid with the GM stuff, you will notice it smells like grapes. Using another fluid besides GM's could be risky as far as the warranty is concerned.

yes, my TB is getting to the point where I want to change all of the fluids, so I was looking at AMSOIL Synthetic Gear Lube. It says it is compatible with most locking rear ends, but how do I know if mine is covered? Also, they sell a diff. additive that is supposed to be able to cover instead of the GM one.

Any input?
Erik

My dealer put some synthetic in my rear end, dont know what name brand it was

shay

My '04 T/B has a limited slip. At 1,000 miles, I drained the factory fluid and refilled it with Mobil-1 synthetic 75W90. I have done this on this vehicle, my '02 Suburban & '99 Tahoe... all with limited slips. I change the differential fluid between 30K & 40K. The Tahoe had 47K when traded, the Suburban has 66k and my T/B 4K. Mobil-1 has graced the differentials and engine oiling systems of all these vehicles and has operated flawlessly. I have seen differential failure due to contamination or vent failure (mounted incorrectly or leaking) or BLD ("boat launch dipping")...submerging the complete differential (vent and all) into the lake. I never used a limited slip additive.

My '04 T/B has a limited slip. At 1,000 miles, I drained the factory fluid and refilled it with Mobil-1 synthetic 75W90. I have done this on this vehicle, my '02 Suburban & '99 Tahoe... all with limited slips. I change the differential fluid between 30K & 40K. The Tahoe had 47K when traded, the Suburban has 66k and my T/B 4K. Mobil-1 has graced the differentials and engine oiling systems of all these vehicles and has operated flawlessly. I have seen differential failure due to contamination or vent failure (mounted incorrectly or leaking) or BLD ("boat launch dipping")...submerging the complete differential (vent and all) into the lake. I never used a limited slip additive.

I'm going to do as you have, run Mobil 1 #75-90 My 04 says it has synthetic oil in it. It has a locking rear end synthetic gear oil is all that is needed, and as long as you use a premium synthetic like Mobil 1, Amsoil, Neo, Redline, etc.

The thing I worry about using a different fluid is GM Spec 9986115. I contacted my dealers (Cadillac and Chevy) and GM customer service (Cadillac) and no one could give me an answer.

Any synthetic 75W90 fluid should not work, but all the owner's manual says is that it must meet GM Spec 9986115.

I changed the rear end fluid today on my CTS V and they did put the additive in. I heard it isn't required on GM Truck's lockers and is not recommended.

I can understand a special fluild for the transfer case but not in the Differential. It like Dodge with their Mopar 4 ATF. My wife owned a Dakota 4x4 before her TB, I took it to Competition Transmissions in Las Vegas. They are one of the oldest and best tranny shops in Las Vegas. the owner serviced it put in Dexron III fluid with a pint of Lube Gard additive. Never one problem, why did I not worry one in the 70s I worked in a high performance tranny shop in San Jose Ca. and we used Ford Fluid in everything, why because it was the best fluid made in those days, Today, Dexron III is the best, on some trannys today you need an added modifier and Lube Gard is the best. The Manufactures for years have been tring to corner the market on repairs and products. One way is tell you you must run this fluid, Like I said, in the 70s Jeep used a special transfer case fluild in there Quadra-Trac, nothing else worked, but rear ends no way. Call Detroit Locker and ask them they are the big boys in locker rear-ends.