1999 C2500 differentail oil

[eddiesbus7401]

Can't find my owners manual, actually don't know if I have one. Going to order one on eBay next. Can anyone tell me what gear oil GM recommends per the manual for my 99 C2500. Most of the parts store show 75-85. I am thinking they show 80-90. It has the American 9.5" 14 bolt. Just looking for a confirmation from the manual. Thanks

 

[df2x4]

The GM recommended oil for all the rear ends in these trucks is GL5 80W-90 with NO friction modifiers or additives. That last part is especially important if you have the factory G80 locker.

This is what I use in my trucks, ACDelco 10-4051. It's one of the only 80W-90s on the market with no additives, at least that I could find.

www.amazon.com/gp/product/B007Q10F3S

 

[Schurkey]

The GM recommended oil for all the rear ends in these trucks is GL5 80W-90 with NO friction modifiers or additives. That last part is especially important if you have the factory G80 locker.

This is what I use in my trucks, ACDelco 10-4051. It's one of the only 80W-90s on the market with no additives, at least that I could find.

www.amazon.com/gp/product/B007Q10F3S

I'm not sure that's correct.

The GM Service Bulletin tells you to use a certain part-number fluid with the G80, and they tell you not to add friction modifier...but they're very ambiguous about whether there's FM already in the recommended fluid. (So there a chance that the quantity of FM already mixed into the fluid is fine, but don't add more FM than what's in the fluid already.)

AAM is no help at all. They list a Texaco-sourced fluid part number that is--apparently--only available in mass quantities of the sort used by an axle manufacturer in production.

And, of course, the P/N for the GM fluid specified has been discontinued, and superseded at least once if not multiple times. I never did track down a suitable "GM" replacement. I did find a GM axle lube with no friction modifier, (the one you linked to) but the back of the bottle specifically said that it was not for use in "trucks", passenger cars only.

I bought some axle lube at the local Home Improvement store a few weeks ago, that says it has no FM in it. ("For top-off only on limited slip differentials, not for complete refill") I haven't driven the truck enough to see how that reacts with the clutches in the G80.

If you DON'T have the G80 Gov-Lock differential, having friction modifier will neither help nor hurt. If you DO have G80 Gov-Lock, I honestly don't know what to tell you, except that "for now" I'm using fluid that does not have friction modifier in it.

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Oh, yeah. The 9.5" axle capacity is supposedly "5.5 pints", which would be 2 3/4 quarts Buy three quarts, make sure you have eight ounces left over.

 

[df2x4]

The GM Service Bulletin tells you to use a certain part-number fluid with the G80, and they tell you not to add friction modifier...but they're very ambiguous about whether there's FM already in the recommended fluid. (So there a chance that the quantity of FM already mixed into the fluid is fine, but don't add more FM than what's in the fluid already.)

Directly from the TSB:

"The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars.
All light duty trucks equipped with RPO G80 make use of a locking differential. The use of additives will delay the engagement of the locking mechanism and may decrease axle life."

The way that's written leads me to believe that there was definitely no additive in the oil that they originally recommended. I've tried looking up images of the label to see but I can't find anything. (Part number 10950849 if anyone else wants to look)

What I do know for sure is that I put an oil with friction modifier in my red truck (10 bolt G80) and it quit wanting to lock the rear up at all. Draining and refilling with the ACDelco 10-4051 I mentioned fixed it.

Regarding the "no trucks" warning on the bottle, I'm not sure what's up with that. But I've been using it in both of my trucks for years now with no ill effects. Best guess, they're talking about big trucks. Like commercial stuff.

 

[Schurkey]

But is it an "additive" when it's pre-mixed with the axle lube?

I've heard lots of opinions on multiple forums, including folks who claimed to work for the various companies involved--but it's impossible to verify. So I'm gonna see how the Valvoline works...if the Gov-Lock engages too abruptly, I'll move to a different lube.

 

[df2x4]

But is it an "additive" when it's pre-mixed with the axle lube?

Fair point and I see what you're getting at, but the language of the TSB and my personal experience leads me to believe that yes, it probably is. At least as far as the author of the TSB is concerned. I'd be curious to read some of the other opinions you mentioned on other forums if you have any links handy though.

I will concede that my first post was over simplified and maybe not entirely accurate (I think some of the later GMT400 open rears may have called for 75W-90 synthetic) but I always recommend GL5 80W-90 with no additives because in my experience it should work with everything. Like you said, friction modifiers won't make any difference with an open carrier, but by that same logic you don't need them with one.

 

[df2x4]

Just for fun I did some digging for more info specifically regarding the OP's question. I wasn't able to find a PDF of a '99 GMT400 truck owner's manual, but I did find one for a '99 GMT400 Tahoe/Suburban. Which should cover the same axles and carriers as the trucks with the exception of the 10.5" 14 bolt full floater IIRC, someone correct me if I'm wrong. Here's a link to it.

https://www.carmanual.org/OwnersManual/chevrolet/1999_chevrolet_suburban.pdf

According to that all '99 GMT400 SUV rear axles call for a specific SAE 80W-90, GM part number 1052271. With a special notation for locking differentials that states "do not add friction modifier." That alone makes me think that the original recommended fluid didn't come with any additives mixed in. That GM 1052271 is also discontinued, but there are a few NOS listings on eBay. For example:

https://www.ebay.com/itm/153917676759

Unfortunately the info on the bottle is practically non-existent. "Limited slip and conventional types" is about all they give you. According to GM Parts Giant that fluid was superseded by part number 89021671, which is similarly vague on the label. It does have one interesting bit though.

www.amazon.com/Genuine-Fluid-89021671-80W-90-Lubricant/dp/B00BK7LUTM

"Compatible with limited slip friction modifiers." Similar to the wording in the TSB, this leads me to believe that it doesn't contain any additives and they want you to add your own as needed. Once again I can't verify that 100% but it seems odd to me that they would list it as "compatible" with additives if it already contained them.

 

[Schurkey]

https://bobistheoilguy.com/forums/threads/does-the-gm-g80-need-ls-fluid.310926/#post-5578276

https://bobistheoilguy.com/forums/threads/fluid-for-g80-rear-end-in-02-silverado.283181/#post-5322222

G80 Gear Lube - Eaton phone call today

Figured I'd share. The net is full of discussion on what type of oil the G80 rear ends need. Syn vs Dino, LS vs non LS. Well I finally got around to calling Eaton's diff tech support. Very helpful guy. He clearly explained the G80 is not supposed to have LS friction modifiers. It does have clutch...

www.gm-trucks.com

Mobil 1 75W-90 in a G80.

www.gmtruckclub.com

I've seen other interesting threads, but I can't find them now.

 

[454cid]

GM switched to the 75w90 synthetic stuff in 99 a few months into the calendar year, if I recall correctly. I think I've seen a date posted before. I'm not sure what my April built truck had in it originally but it's got dino 80w90 in it now....some of which may be 40+ years old.

I can tell you that the 11.5" axle in the GMT-800's used one gear oil, no matter open or locking.

 

[df2x4]

https://bobistheoilguy.com/forums/threads/does-the-gm-g80-need-ls-fluid.310926/#post-5578276

https://bobistheoilguy.com/forums/threads/fluid-for-g80-rear-end-in-02-silverado.283181/#post-5322222

G80 Gear Lube - Eaton phone call today

Figured I'd share. The net is full of discussion on what type of oil the G80 rear ends need. Syn vs Dino, LS vs non LS. Well I finally got around to calling Eaton's diff tech support. Very helpful guy. He clearly explained the G80 is not supposed to have LS friction modifiers. It does have clutch...

www.gm-trucks.com

I've seen other interesting threads, but I can't find them now.

I read over those quickly, thanks for the links. As a disclaimer I have no idea if GM continued using the Gov-lock G80 carriers that we got in newer vehicles, but I do know that the G80 RPO was used with several different types of locking and limited slip carriers over the years. For example, the clutch type posi in '90s Camaro/Firebirds. What I'm getting at is that not all G80s are created equal, and I bet that's where a lot of the confusion comes from. I saw some others mention that fact in those threads as well.