It is time for me to change the rear gear oil (grape juice). Do I have to put this back in or may I use any syn gear oil ?

Thanks
Mark

I went with the grape juice.

It is time for me to change the rear gear oil (grape juice). Do I have to put this back in or may I use any syn gear oil ?

Thanks
Mark
mobil 1 in mine.

4DAMAX, the truck comes with synthetic gear oil in it from the factory. IIRC the Eaton folks call for the GM brand of synthetic, sumpin about special additives or something like that. I had mine changed out with the heavier GM synthetic since I tow pretty heavy. Now I have a rear axle seal leak which is being fixed tomorrow. :o:

If there isn't a lot of difference cost or performance wise I stick with GM's stuff. Kind of hard for them to fight me on any warranty claims when it is their stuff in there. Later. JJ :)

Amsoil Severe Gear 75W90 in mine. No problems.

JJ, The folks at Eaton recommend a GL5 rated gear lube, not specifically the GM grape juice. There are numerous long threads here on the very topic and at least 1 contains direct e-mails from Eaton on the very issue where they state GL5, they do not specifically call for the GM brand stuff at all.

The following was posted on an earlier thread where gear lube was discussed. Backs up what Hound stated. The GM brand is the only one they have done extensive testing on, that's why I thought they recommended them. My bad, good intel!

"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.
¨ Note: the recommended lube for HD trailer towing is a 75W-140 Synthetic Gear lube, part number 12346140.
III. Inspection/Change Frequency
¨ The 1500 and 2500 GM Trucks recommend rear axle fluid level checks every 7500 miles. Fluid should be added as needed.
¨ Under heavy duty trailer towing conditions the lube should be changed after the first 500 miles of towing. This is due to the extreme heat generated on break-in of the hypoid gear set."

[ <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><st1:date Year="2002" Day="26" Month="10">10-26-2002</st1:date>: From TxDoc over on the dieselpage ]

And one last note: Please use synthetic gear oil.<!-- / message --><!-- sig -->

This was provided by the guy that invented the Eaton locking differential axle. Good discussion folks. JJ :)

So to answer your question you dont necessarly have to put the high priced grape juice back in, mobil, amsoil, all good too, as long as its synthetic.

This was provided by the guy that invented the Eaton locking differential axle.
If you mean the Gov-lock (Gov-junk, shat-lock, time-bomb locker, etc) I'd like to wring his neck...

I've had more than my share of G80s bite it in 1/2 tons...

forgot gov-bomb and grenade-lock

I used mobil 1 75 90w

Sorry Phil. Thanks for picking up my slack on the derogatory G80 names ;)

You do realize that the locker in the rear of a 1/2 ton is signifigantly different than the one in a 3/4 HD truck.