Hi, I found out that GM often underfills the rear diff on new trucks (from another thread on this great site), so I ran out and checked mine and it's a good 1.5 inches under the fill hole. The truck only has 860 miles on it, so I'm not ready to drain and refill the rear diff yet. I have used Redline synthetic gear oil in the past and was wondering if I could just top off the diff with some 75-90w redline instead of buying a quart of the GM stuff. Thoughts? Thanks...

Change It.

Over 500 miles, change it.

MHO.

Idahofox

I towed my trailer (10K#s ) for a about 100 miles after 500 then changed with Royal Purple. Aso changed the front and the T-case after some time (5-6 hours) in 4WD for piece of mind.

How about asking your dealer to provide 1 qt of GM gear lube. The stuff lists for $29.00/qt. Then change to fluid of your choice at some point in near future, this way there is no compatability issue. The rear axle fluid is to be changed at the first 5000 mile mark. Mine was so filthy I wish I had changed it sooner. I used Amsoil Severe Gear 75w90 product code SVG and our G2039 Hand Pump to install. The drain plug on the bottom of rear axle housing makes draining a breeze.

I'd go with the recommendation to change it. Eaton recommends an early change after the first 500 miles because there is significant "run in" on the ring and pinion that produces metallic debris and generates enough heat that it can degrade the lubricant. I found a considerable amount of metallic "paste" on my drain plug magnet and even a few metal shavings at the 500 mile change. Of course the dealer will want to sell you the "grape juice" for $29.00 a quart, but any good over-the-counter synthetic lube will provide you good protection and great service. I've been using Amsoil since the 500 mile change, many have been successfully using Mobil 1, Redline and others.

I would vote to just change it. The total cost of any of the other synthetics will probably be less than the cost of a quart of the GM stuff,... and as someone else on this form noted earlier, if it costs $29 a quart it will probably be reserved for me, not the truck.):h
BTW the drain plug is a 3/8 square drive. I learned the hard way two days ago when I changed mine. After dragging all of my allens, both sae and metric and torx, I finally got the mirror and checked. Just use the ratchet, it works fine.:grd:

Thanks guys. I was planning to change it early, but I haven't towed with it yet, and I understood there to be additional run-in when you put these things under load -- true?

The dealer I bought it from is a good distance from me, but a local dealer said they'd check it and top it off if needed today, tho they did say this was missed on the selling dealers PDI, so they'll get back charged for it...

I'm going to try to get a hundred or so tow miles on it and then do the change-out -- should be at about 1k miles then... (along with changing the Ally filter and Dmax's oil/filter then too :) )