75W-140 Synthetic Rear Diff Lube

[LittleBigRig]

My 2001 owner manual only specifies one synthetic lube (75W-90) for the G80, but I hear rumors that later model owner manuals also listed a second synthetic lube (75W-140) for heavy trailering applications. However thus far, all I can find is a thread listing a non-GM recommendation from the company that tested the axles.

Two questions:

1) Can anyone substantiate the rumor by listing the actual year model owner manual making this alternative recommendation, and if so

2) would you be so kind to provide the GM part number shown in that owner manual?

Thanks in advance!

[LanduytG]

It all depends on how much weight are you going to be towing. If in the 18K and over the 75W-140. If less the 75W-90 is OK.

Greg

[mdrag]

LittleBigRig,

IIRC, most feel that the 75W-140 is NOT needed for the G80 and a synthetic GL5 75W-90 is more than adequate.

Here is a quote from TxDoc's post (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=7;t=002814) in a previous topic, with GM part numbers listed for both lubes:

[b] [quote]"The post I made was a quote directly from Eaton's reply about what is okay and not okay for use in conjuction with the locking rear end option.
Back in November, I received the following:
"From Mr. Ralph Holmquist of Eaton, the maker of the locking differential:
"The maintenance schedule for the rear axle was developed by American Axle &
Manufacturing and GM truck based on multiple tests. The Eaton locker does
not require additional maintenance nor does it add heat to the lube. The
lube will darken due to the carbon wear on the clutch surfaces, much the
same as a disc brake pad & rotor. This does not damage axle components such
as seals or bearings. However, a new axle can produce excessive
temperatures (plus 350 degrees F) due to the ring & pinion breaking in that
will break the lube additives down. Avoid high loads, trailer towing and
high speed extended driving during the initial break in of the vehicle.
After the break in period axle temps will level at a much lower figure.
Lube changes are a good idea because the additives are replenished and
contaminates such as casting sand are eliminated. The axle is filled at the
factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276.
The GM service # is 12378261. This is the only lube we have done extensive
testing with to insure locker compatibility. The only negative to using one
of these other lubes is an increased potential for clutch chatter. This
really doesn't hurt anything and can be corrected by changing the lube. I
noticed in the latest GM owners manual the term "or equivalent" when
referring to the lube specification. Look for a GL5 rating on the bottle to
make sure the ring & pinion, seals & bearings are protected. Limited slip
additive is not needed."
The following information outlines the type of
rear axle lubrication utilized in GM Truck axles.
I. Fluid Type
II. The 1500, 2500 and 3500 GMT 800 Trucks utilize SAE 75W-90 Synthetic Axle Lubricant. The GM part number is 12378261 and the specification is 9986115.

[LittleBigRig]

Thanks guys for the advice and the reference, but I'm looking specifically for the GM recommendation in an owner's manual. TXDoc's reference is the "non-GM" recommendation from the company that tested the axles that I referred to in my original post.

I tow a 12,000 trailer through some tough grades on a regular basis. Trust me, this is a heavy-duty application that pushes the advertised (and warranted) design limits of the truck.

Thanks again for your support!

[jjackson]

LittleBigRig, I tow a 13K 5th wheel, and after changing the rear diff fluid at 500 miles, I went with the Amsoil 75-140 synthetic fluid.

I don't have my owners manual with me, but I believe the 2003 book makes that recommendation.?

Certainly couldn't hurt to use the heavier fluid.

John

[Lawnboy]

I would think the only thing that would suffer by using the 75-140 is your unloaded, cold fuel milage.

The thicker viscosity is harder for your ring gear to spin through, thus creating drag.

Though in your high GCWR and the amount of towing that you do, I wouldn't hesitate to use 75-140 at all.


Tomorrow, I will be able to tell you what an '04 manual says :D Getting a new truck!

[Inspector]

Tom Jr.

Oh are you going to make your Dad jelous. Last year your Dad and Howie and I carvaned to the rendezvous in Missoula form here in Layton Ut.
We had a great time. Maybe we will see you at #6.
Denny

:D