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Re: Fluid for G80 rear end in 02 silverado? [Re: Rhymingmechanic] #4655979 02/04/18 02:07 PM
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WishIhadatruck Offline
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Originally Posted By: Rhymingmechanic
Originally Posted By: DoubleWasp
G80 diffs must have changed at some point. All of mine have metal warning tags on them that additive must be used.


My understanding is that G80 is an option code. Depending on the axle or vehicle, that could mean the mechanism is either a limited-slip or a gov-lock.

One of my earliest posts was about the arcane GM spec number for axle fluid in my 2001 Silverado. It had the grape-scented special sauce from the factory. I ended up replacing it with Amsoil Severe Gear 75W-90, and the locker has continued to work for the last 70,000 miles.


Correct, both my Suburban and Roadmaster both have G80 option codes. They are completely different axles and differentials. The Roadmaster needs LS additive, the Suburban does not.


1996 Buick Roadmaster Wagon
2014 Cruze Eco Manual
2003 2500 Suburban, 8.1L
Re: Fluid for G80 rear end in 02 silverado? [Re: Sealbilly] #5322157 01/14/20 04:15 AM
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dave1251 Offline
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I use Chevron DELO 75W90. No complaints.


make the inside of your engine oil cap white.
don't use.
Re: Fluid for G80 rear end in 02 silverado? [Re: Sealbilly] #5322222 01/14/20 07:11 AM
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dnewton3 Offline
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I had looked into this topic DEEPLY many years ago when I had my Dmax/Alli with AAM 11.5" axle.
Always go directly to the source!
https://www.eaton.com/us/en-us/catalog/differentials/mlocker.html
The G80 most certainly has clutches ...
" Utilizing a flyweight mechanism and self-energizing clutch system ..."
Here's the unit in a video; great explanation
https://www.youtube.com/watch?v=ftyJvIO0DZ8
Note they call they clutches "active friction disc" and such ...


The G80 requires a fluid with friction modifier, pure and simple. It is a clutch-driven limited slip unit. The reason they call it a "locker" is because it uses a fancy flyweight system to "lock" the clutches (essentially by just clamping down tighter on the clutch pack) to increase torque bias. The unit is ALWAYS in a LS mode; it only "locks" when a preset amount of wheelspin causes the clutches to lock up in a tighter grip. Hence, it's a fancy version of a LSD. And it needs some FM to work correctly.

There is an old document from GM that states to NOT add FM to the fluid. This is a misleading statement. They (GM) sell their own diff fluid (affectionately known as "grape juice" because it has an odd odor of grape juice) and the over-the-counter product already has the FM in it. And because it is just as bad to have too much FM as it is to have too little FM, GM's position is "don't mess with our fluid; don't add FM". But it's really a matter of viewpoint. They are telling you not to add FM because they already add FM to the bottled product. What they should be saying is "Don't add any more FM; we already did it for you." The GM part number for their lube is 9986115. If you search old documents about it, you can read more. Amsoil even came out with a document a long time ago explaining the whole FM topic regarding GMs product, but I cannot find a copy of it at the moment.

If you use diff fluid in the G80 that has no FM whatsoever, you will get clutch chatter and nuisance locking. Adding about 4 oz of FM to the fluid will be a decent place to start. Then add a tiny bit at a time until those nuance and chatter events go away.



More than a decade ago I had a email conversation with an engineer at Eaton about this differential. I would told that all it needed was a GL-5 lube in a typical grade (dino or syn, 75w-90 up to 140 grade). Add in some FM if the fluid does not include it. Done.



Last edited by dnewton3; 01/14/20 07:25 AM.

The act of preventative maintenance, in and of itself, is FAR MORE important than brand/grade/base choices among lubes and filters.
- under maintaining something is akin to abuse/neglect; that can kill equipment by shortening the lifespan
- over maintaining something has never been proven to be anything but a waste of time and money
Re: Fluid for G80 rear end in 02 silverado? [Re: dnewton3] #5322252 01/14/20 07:53 AM
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N Heat Offline
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Replaced the "grape juice" in my 02 after 50k. Ive used Mobil 1 75w90 and made it to 270k.

Re: Fluid for G80 rear end in 02 silverado? [Re: Sealbilly] #5322519 01/14/20 02:18 PM
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dave1251 Offline
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I use DELO 75W90 withy additional additives in my Eaton locker.


make the inside of your engine oil cap white.
don't use.
Re: Fluid for G80 rear end in 02 silverado? [Re: dave1251] #5322534 01/14/20 02:39 PM
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4WD Offline
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Easy to find confusion for years … look what this guy claims Eaton told him regarding the MLocker

AAM and GM (other OEM) also get a vote on lube specs

https://www.gm-trucks.com/forums/topic/183167-g80-gear-lube-eaton-phone-call-today/

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