GM rear differential lube - synthetic 75W-90...

[NewGuy]

the synthetic gear lube for my Tahoe costs $28/qt at the dealership. It has a limited slip differential that seems to require a special synthetic gear lube.....but $28/qt seems very unreasonable.

Does anyone have experience with this application? The GM part # for the gear lube is 9986115.

Thanks for your help!!

 

[schwartr88]

Valvoline synpower gear oil is supposed to have the limited slip additive in it, and its 1/3 the price.

 

[gtx510]

Synpower and M1 will both have the LSD additives.
But I find that I can get Redline, Royal Purple, Amsoil, Specialty Formulations, Motul, or Liqui-Moly for less $$.

GM and Redline also sell the LSD (FM) Friction Modifier additive separate.

 

[Pablo]

What year is your Tahoe?

GM Synthetic lubricant, GM part no. 12378261, specification No. 9986115 is what is called for in 2005 and similar.

Amsoil recommends:

Amsoil SVG 75W-90

I give free shipping to BiTOG members.

 

[NewGuy]

Pablo.....I believe that is the specification and part # required in my owner's manual. Would you like to PM me a price?

 

[Rhymingmechanic]

I went around and around trying to find a gear oil that satisfied the same spec, and finally ended up ordering some SVG from Pablo. The Amsoil seems to be doing just fine.

My best guess is that the GM oil is "special" because it has grape flavoring. The truck rearend does not require LS additive like a Camaro of Corvette. I have seen the GM grape juice at an AC Delco store for significantly less than dealer price, but still pretty steep.

 

[NewGuy]

Rhymingmechanic....I think you might be right about that AC Delco gear oil. My Tahoe has about 75K on it now and I was thinking that it was just time to replace the gear oil. I called the Service Dept at my local dealer and they don't recommend changing the gear lube until about 100K miles. What mileage did you truck have when you changed your rear diff?

Also, did the Amsoil material come in a squeeze bottle like the M1 material? The picture on the website had a non-squeeze bottle.

Does anyone have any comments on running the rear differential fluid for 100+K?

 

[Pablo]

Amsoil is not a squeeze bottle, but the common inexpensive hand-pumps and spouts fit the large mouth bottles.

100K might be pushing it, but people do it all the time.

 

[cfromc]

I ran the "never replaced just top-off" gear lube in my 2001 Suburban for 90K miles and it came out looking pretty darn good. I got swayed by the owner's manual warning that no other fluid should be used so I mixed 1 quart of the $29 a qt GM stuff and 1.75 qts of the Amsoil 75W-90. It's been almost 30K now and no problems.

The front diff OTOH, with it's conventional fluid 80W-90 and required service intervals, looked really, really nasty when I took it out at 90K (unsure of previous maintenance history on the front diff). The front diff fluid was gray and had a LOT of metal in it. I used Amsoil 80W-90 and replaced it again at about the 110-115K mark. While it wasn't gray or nasty looking, it did have metal particles in it (nothing like the first time though). Hopefully now it has been flushed out and when I change it at 150K it will be clean. Goes to show the difference between synthetic and conventional. The front diff rarely has power through it (use 4WD maybe twice a year) yet looked horrible. The rear diff always has power through it, is never recommended to be changed, yet looked pretty good. The GM stuff has proven to me to be very good but is sooo expensive. If someone can reassure me that it does not need to be used at all, then I will go 100% with a $10/qt fluid. Also, I'm thinking about going with a 75W-90 in the front diff. I don't see why it needs an 80W-90.

 

[NewGuy]

This is all great advice. Thank you.

I have one more question....how many quarts does the rear diff hold? I am thinking about 3 quarts. Does anyone remember? The owners manual isn't any help.

thanks again!!

 

[Pablo]

Differential, with 9.50 ring gear Rear.......NA[4]
Differential, with 8.6 ring gear Rear........NA[5]

4. Fill no higher then 0.4" below plug hole.
5. Fill no higher then 0.6-1.6" below plug hole.

No help! Maybe 3 qts!

 

[cfromc]

Per my owner's manual it is 2.75 qts. Check your owner's manual and try to fill it up to a little over the middle of the specification. In reality it takes about 2.5-2.7qts.

 

[NewGuy]

I just ordered 3 quarts of Amsoil SVG 75W-90 from Pablo. I will find one of those twist-on spouts and fill with 2.75 quarts when the material arrives. Thanks for all of the help. I will report back on how things go.

 

[cfromc]

Well, just because the spec says 2.75 doesn't mean that is right for a maintenance drain and fill. Use the level spec. What I did was got some pipe cleaners and started with 2.5 quarts and then bent the pipe cleaner and put it in the hole. Then I pulled it out and measured where the fluid was in relation to the bend. My spec said between 5/8" and 1 5/8" from the hole. I filled it to about 1-1.25" from the hole. This is smack dab in the middle of the spec. Too little and no margin for error in case of any fluid loss, too much and too much drag on the system. It is likely that 2.75qts will be too much.

 

[Rhymingmechanic]

Newguy,

I don't have the numbers in front of me, but it seems like the change was at or before 50,000, about a year and a half ago. The original fluid was black and smelled like grape Kool-Aid.

The Amsoil bottles have a wide mouth, like a gallon jug. I used a twist-on/off spout with a clear vinyl hose (from a parts store) to install the fluid--there was plenty of room under my pickup to do it that way.

Anyone interested can search this forum for the "9986115" and find out quite a bit about the workings of that differential, synthetic fluid compatibility in front differentials (I think there is a GM TSB about vent hose materials), and what the spec. may or may not mean. Somebody even posted photos of their drained fluid.

 

[gtx510]

How is overfilling a diff harmful?

Most people try to overfill a diff or manual g/box.

 

[NewGuy]

The Amsoil SVG 75W-90 gear oil arrived this afternoon. I can't believe how fast they responded. I am going to drain the rear diff tomorrow and re-fill.

The rear diff does not have a drain plug, so I assume that I will loosen the bolts on the diff and pry it open with a screwdriver to drain. Then I will tghten everything back up and fill through the plug on the top. Should I worry about the rear diff leaking at all? I don't need to replace a gasket or anything do I?

Thanks for your help.

 

[ConfederateTyrant]

NewGuy,
When removing the diff cover, you will need to replace the gasket, or use RTV sealant. I have used gaskets in the past on Fords without any problems. Advance Auto, Auto Zone, etc. should have a gasket in stock, but some prefer the RTV.

 

[cfromc]

quote:

---

Originally posted by gtx510:
How is overfilling a diff harmful?

Most people try to overfill a diff or manual g/box.

---

It could cause leaks and it will likely increase the temperature of the unit due to the increased drag. Also, it is just not needed and has no benefit.

As far as gasket vs. RTV, usually using a gasket works better. If you have the repair manual, follow the specs for putting the cover back on. If not, I would use a criss-cross pattern and don't over-tighten the bolts-if they stop turning because you've got them so tight, that's too tight.

 

[acewiza]

Measuring the additional drag from an extra 6-8 ounces of fluid in a rear would be like measuring the difference in the speed of light as it passes through the atmosphere as opposed to outer space.

One function of the fluid (besides lubrication) is cooling. The additional fluid will dissipate heat faster and help cool the diff better, NOT make it run hotter.