Mobil 1 75W-90 in a G80.

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by stchman, Dec 21, 2012.

  1. stchman

    stchman Active Member 2 Years 1000 Posts

    I am wanting to change out the differential fluid on my 10 bolt G80. Has anyone here had issue with using Mobil 1 75W-90 gear oil? GM wants ~$30 a quart for their part # 89021677 fluid.

  2. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

  3. j cat

    j cat Epic Member 5+ Years 1000 Posts

    since 2000 I have used the recommended mobil synthetic 75=90 wt gear oil in the G80 eaton locker rear dif with no issues.

    remove the rear cover clean out box and the cover magnet use mineral spirits paint thinner and a cloth.

    use sealant on the cover place gasket on the cover coat gasket with sealant. let sealant skin first 30 min then install cover to the cleaned diff flange.

    fill to 1/2 in below the fill plug . go for a test drive and recheck. sometimes you can get a drop in the oil level after operation.
  4. stchman

    stchman Active Member 2 Years 1000 Posts

    Vry interesting thread, I enjoyed reading about the mechanic that thought conventional gear oil allowed junk to "stick" to the differential housing. I guess he wanted to completely disregard the fact that GM calls for a synthetic 75w-90 gear oil by name.

    I am seeing at of people that swear that you should add LSD to the oil as it will help the G80's clutches. The G80 is a locker, not a limited slip.

    I feel that using Mobil 1 75w-90 is a good choice.
  5. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Actually, it's a limited slip with a locker (or, at least, that's how GM describes it); but, I don't think it needs additive either

    I agree.
  6. steved

    steved Former Member

    The G80 parts diagram I've looked at make it look like it is basically a clutch style LSD with the ability to force the the clutches together when it senses slip between the two wheels. It actually doesn't lock in the sense a true locking differential does (like a Detroit Locker, which has no clutches).

    Further, not every LSD requires additive...the Trac-Rite in my Dodge didn't.
  7. Rockstar 100 Posts

    On my Colorado we just swapped in a 2006 Trailblzer SS rear axle assembly. 9.5 Eaton locker 4.10 33 spline axle. We used Mobil 1 75w 90 oil and we added 4 ounces of the additive. The dealer actually gave me the TSB where it states that (this particular) rear axle can develop "howling", and early wear on the clutches if the additive is not used. He was nice enough to let me know about it, so I followed his advice per the TSB.
  8. j cat

    j cat Epic Member 5+ Years 1000 Posts

    the additive of the friction modifier has been not recomended in the past because it will damage the G80.

    post the TSB # or reproduce this here.

    the only issues I have found with a new eaton G80 is it requires a breakin period with light loads for the first 1000mi. then after 2,ooo mi the rear cover is to be removed the gearing teeth inspected for proper meshing and the boxed cleaned.

    I have read many documents with the G80 warning not to use any friction modifier additive.

    If you drive this vehicle hard the first 1000mi the gear box will get very hot and then the teeth will be brittle and crack and break off. no loads as well..

    - - - Updated - - -

    MR GM TSB Said:
    Some light duty trucks equipped with locking rear axles (G80) may exhibit rear axle chatter, especially when turning a corner from a stop.
    This condition of alternate engagement and disengagement of clutches in differential assembly is usually caused by contaminated axle lubricant.
    To correct this condition, drain and refill the rear axle with SAE 80W-90 GL5 (P/N 10950849).

    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 and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.

    VEHICLES/COMPONENTS INVOLVED: ------------------------- ---- Some light duty trucks equipped with locking rear axles, RPO G80.
    Part Number Description ----------- ------------------ 10950849 Lubricant, Rear Axle (1 litre)
    Parts are currently available through CANSPO.
    As specified in Light Duty Truck Maintenance Schedules, locking rear axle fluid drain and refill is required owner maintenance at the first engine oil change. Failure to drain and refill the rear axle as specified may contribute to a later axle chatter condition. Refer to the appropriate Light Duty Truck Maintenance Schedule or service manual, section OB, for further details on change intervals.
    General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.
    © Copyright General Motors Corporation. All Rights Reserved.

    as you can see a clean box causes the diff to work as designed. issues arise when it is not clean. this causes delay in engagement which wears out the diff.

    also no additive as this will damage the G80 diff.
    SAE 75W-90 Synthetic Axle Lubricant, GM P/N 12378261, this is the correct gear oil to use . the other gear oil stated by GM was to correct chatter issues IF the box was not dirty and still chattered.

    good luck with your repair.
  9. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    The Trailblazer SS trucks do not use the G80, they use a true LSD diff, additive is probably a smart move.
  10. j cat

    j cat Epic Member 5+ Years 1000 Posts

    Document ID# 1870450

    Subject: Growl groan Shudder or bind from the rear differential when turning - keywords axle chatter cold G86 hot turn #PIP3911A - (11/03/2006)

    Models: 2006-2007 Chevrolet Trailblazer SS Equiped with limited slip differential (RPO G86)

    Condition/Concern: Same as Subject


    Replace the rear differential center section (gear case) which will inlcude clutch plates and springs with an updated spring rate. At the time of the release of this PI all parts in GMSPO stock have been purged and ordering the catalog listed part number will insure updated parts. Fill the differential by first installing 5.5 oz (163 mL) of Limited-Slip Axle Lubricant Additive (GM Part No. U.S. 1052358) then fill, 0.6 in. to 1.6 inches below th ebottom of the filler plug hole using SAE 75W-90 Synthetic Axle Lubricant (p/n 12378261) meeting GM Spec 9986115.

    Note 1: reference the latest version of tsb 06-04-20-003 for proper fluids and capacity.

    Note 2: Synthetic Axle Lubricant, p/n 12378261 or p/n 89021677 each of these part numbers is the same axle lubricant, w/ differences in packaging only.

    Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved w/o completing every step, the remaining steps do not need to be performed.

    this is the TSB . the diff in a trailblazer ss is the G86 .

    quite different than the G80 in our trucks.

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