I was just going over my service schedule and found
this gem in my records........

Written by "Shawn Spickler" of Off-Road.com

The greatest damage to a new gear set results when it
has been run for ten minutes or more during the first
500 miles and the oil is very hot. Any heavy use or
overloading while the oil is extremely hot will cause
it to break down and allow irreversible damage to the
ring & pinion. In order to make them run cooler and
quieter, new gears are lapped at the factory. However,
they are not lapped under the same pressures that
driving creates. These loads generated while driving,
force any microscopic high spots on the gear teeth back
into the surface of the metal. This is called "work
hardening". Work hardening is similar to forging in the
way that it compresses the metal molecules into a very
compact and hard formation. This can only be
accomplished if the metal surfaces are lubricated and
the gear temperature stays cool enough that the
molecular structure does not change. If the temperature
of the metal gets hot enough to change the molecular
structure, it will soften the surface instead of
hardening it. This may seem like a balancing act but it
all happens easily & passively as long as the oil keeps
the gear cool while it is breaking in. Some of the
synthetic oils on the market today can help a gear set
live longer. Theses oils will continue to lubricate at
temperatures where many crude oils break down. Even
with synthetic oils, you should still follow the
procedure for BREAKING IN A NEW GEAR SET:
After driving the first 15 to 20 miles it is best to
stop and let the differential cool before proceeding.
Keep the vehicle at speeds below 60 mph for the first
100 miles. You should put at least 500 miles on the new
gear set before heavy use or towing. During the first
45 miles of towing it helps to go about 15 miles at a
time before stopping to let the differential cool for
15 minutes before continuing. This is necessary because
not all of the gear tooth is making contact until it is
heavily loaded. When towing, the teeth flex to contact
completely, and cause the previously unloaded portion
of the teeth to touch and work harden. All of this may
seem like paranoia, but it is very easy to damage the
ring & pinion by overloading before the teeth are
broken in. It is a good idea to change the gear oil
after the first 500 miles in order to remove any metal
particles or phosphorus coating that has come from the
new gear set. This is cheap insurance and a good time
to discover any problems before they grow to disastrous
proportions.
Again thank you Shawn Spickler

"the teeth flex to contact completely"

That's cool.

The 500 miles rear end change is in our owners manual.

I thought I read somewhere on mine that said "Extended Life Rear Axle Fluid" or something along that line, which for an average owner that would mean you could put quite a few miles on it before you need to drain your rearend. Might be kind of confusing.

AAM are the folks that recommends changing the oil at 500 miles not GM or Eaton. Later! Frank

I am not sure how hot my rear end gears got when I first purchased the truck, but I think the real problem is metal peices floating around in there and this will happen even if the temps are below the required limits.
How does this effect the front end? If I remember correctly they do not say anything about changing the front end gear lube as it does the rear end. If you buy your truck in the winter time and have it in 4 wheel drive for the first 500 miles, wouldn't this be just as prone to damage?

I just got the computer reflashed today. While talking to the mechanic, I asked the same question about diff fluids. He experienced more front ring/pinion failures due to metal shavings. I will do both front and rear, but at $25/qt for GM's synthetic, it's a little pricey. Mechanic recommended at least checking the drain plug with magnet at bottom of diff for shavings and a little fluid dropped onto rag to check for same. Don't know by experience, but it can't hurt to just change both. 8,000 miles too late...

I was at the dealer @ 5k and they recommended I change the rear fluid. I agreed... and after a few days also wondered about the front differential. Seems like a good idea to do both to me now that I think about it?? My dad had his D/A at the dealer this year at 5or6k for all of its fluid changes and they told him NOT to change the gear oil-go figure.

If you go to expense of changing the dif fluids you should also change the tranny and the transfer case. They have the same charectoristics. Metal grinding on metal.

I waited to change my rear dif fluid after 2,000 miles. About 400 miles of this was towing a 7,000 trailer. I changed to a Mag-Hytec cover with the magnetic dipstick and magnetic drain plug. I put in Amsoil series 2000 75W-90 synthetic gear lube and have driven another 37,000 miles of which 3,500 miles is towing. I check the dipstick once per month and wipe it clean of fuzzy metal filings. The light blue gear lube was still fine but when I unscrewed the bottom plug out, out came the nasty fuzzy stuff at the bottom of the differential. I used BrakeKleen to flush the bottom out, secured the bottom drain plug and filled her up with Amsoil's new Severe Gear 75W-90 gear lube. This gear lube is a familiar honey colored. I have a little over 1,000 miles since the change and need to check the dipstick.

I plowed with my truck the night I picked it up. I think it had 150 miles on it. It is up to 9000 mile with original lube.

I have 109,000 miles on 1 of my trucks without ever changing it. Towed with it the first day I got it, and almost every day since. No problems here, except now that you guys have reminded me I will change it soon. One of those things thats easy to forget. Religous about engine maintenance and tire rotation, just forgot about rearend.