Rear diff fluid for 08 Silverado - Mobil1?

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Jun 12, 2009
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Nashville, TN, United States
I need to change the rear diff fluid on an 08 Silverado and 07 Avalanche, both with the G80, so it's not a limited slip. The GM spec for the fluid is 9986115. But, after looking on the back of all the different gear oils, none of them say they meet this specification. On the GM forums, a lot of people are using RP or M1 75w-90. All of them say they exceed GL-5 specs. Is it ok to run the M1 in my truck without any issues? From what I've read, it seems like it should be ok. What are your thoughts? Also, I thought I read somewhere that Mobil1 makes the GM gear oil. Is this true?
 
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Jun 19, 2007
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MS
Why do u want to change it so early ? I think GM already use good stuff on it and it should last a very very long time. Mobil product is what GM factory fill.
 
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Are you still under warranty ? anyway Mobil 1 sound great. Or AMSoil is also exelence, btw what ever syntetic gear oil that say already had the Additive, just add another bottle of GM Additive to it.
 
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If the truck is a 4x4, change the front diff. & the transfer case - that is where I found most of the shavings.
 
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 Originally Posted By: killagt
Why do u want to change it so early ? I think GM already use good stuff on it and it should last a very very long time. Mobil product is what GM factory fill.
The idea is to remove the factory fluid and all of the break-in metal it contains.
 
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Ran Mobil 1 75W-90 in my 97 Chevy fullsize 5.7 liter 4WD and never had an issue in 110,000 miles/10 years!
 

JHZR2

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M1 or Amsoil SVG. I like the SVG as you can get a 75w-110 and have a bit heavier oil without much sacrifice.
 
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Jul 7, 2007
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That part # is for the GM 75W-90 synthetic Grape Juice. Another overpriced GM OE fluid. I am sure the Mobil 1 would be ok. I had a 2005 Silverado with the G80 and a 2007 Silverado( NBS - same as your 08 )with the G80 and ran RP 75W-90 front and back. Worked great.
 

dnewton3

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I did some research on this GM "grape juice" a couple year ago. Here's a synopsis of what I can share. Make sure to note the there are "spec" numbers, and there are "part" numbers. The spec is for the fluid; the part is for the packaging. GM #9986115 is a spec for the blended product of GL-5 hypoid gear oil (of synthetic base stock) and some friction modifier. Why FM in a "locker" rear end? Because there is a small actuating clutch that moves the locking pawl mechanism, and that clutch can "chatter" at times. The FM quiets the operation of that clutch. It will not be hurt if you don't use FM, but it can be anoying both with audible and physical nuisance locking. GM apparently buys the hypoid oil in bulk, and the FM in bulk, and mixes them at the plant? (Best info I could find at the time). So, to make the "grape juice" availabe over the counter, they had to create a new spec (9986115) for the "blend" of oil/FM, and then make it available in a bottle for sales over the parts counter. (there were, at the time, both pint and quart bottles, so the part numbers you may see quoted will vary, depending upon container size). The reality is that the "grape juice" diff fluid from GM is nothing special; it's a "synthetic" GL-5 with FM. That's all. It performs reasonably well, unless you consider that it's stupid-expensive compared to its competitors such as Mobil 1. Here is a link to a conversation regarding some information straight from some tech guy at Eaton, the manufacturer of the G-80 locking units. In a nutshell, they used "Texaco" brand fluids for testing, but he clearly states that multiple grades and base stocks can be used, as long as they meet GL-5. Read where he calls out both dino and synthetic options for the GM fluids. If you'll peruse down about 1/2 way, you'll see a topic for "automatic locking differential lubricants" Here's the link: http://www.swedishbricks.net/700900FAQ/Driveline1.html Amsoil also has similar info in their TSB #DT-2004-07-01. Hope that clears it up. Use any GL-5 you want; you'll be fine. Use "synthetic" if you want longer fluid intervals.
 
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Dec 2, 2002
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Washington St.
Don't think that GL-5 is anything special. That spec has been around almost forever. It means that the gear oil handles the sliding motion of hypoid gear teeth. Hypoid means that the input and output are in different planes. Notice how the height of the input is lower than the output--that's hypoid, and that requires GL-5. It appears that differential makers are specifying synthetic gear oil in certain modern differentials because the oil handles higher loads and temperatures before the oil breaks down (and when the oil breaks down the parts fail very soon thereafter). The differentials are being made smaller, lighter, and cheaper, and relying on more robust gear oil for their survival. During the warranty period, I'd strongly suggest that you stick with GM's distinctive gear oil. If you have a warranty claim, you don't want anything to show that you used an non-approved product. (And, no, I do not consider Amsoil's warranty equal to any equipment manufacturer's warranty...read all of Amsoil's fine print about how to qualify for reimbursement.)
 
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Jan 31, 2006
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 Originally Posted By: AzFireGuy79
...............Mobil 1 makes a very high quality gear oil.
If this is true, why did M1 shear out of grade after only 10,000 miles in my Subaru?
 
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Jan 3, 2009
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I use it in my Merc with no problems, and my sons Ranger has it in his diff. also no problems. 333,000 miles on two diff oil changes.
 
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