"GM introduced the new body style in 1998. At the same time they upgraded
the differential and changed to a synthetic lube. The G80 is specifically
designed to give ultimate performance with this lube, and the lube contains
special additives that help reduce slip stick that occurs between the
clutches. the GM synthetic lube typically runs about 100+ degrees cooler
than mineral base lube, which is better if you do allot of towing or heavy
hauling. I would recommend that you stay with the GM synthetic, not Mobil 1.
If you don't tow or haul you could use the pre 1998 mineral lube without
degrading performance of the differential. this would be a cheaper option
but require more lube changes. "

"Axle tolerances of the 1/2 ton trucks are very tight. The recommended
procedure was to change lube after 500 miles of heavy towing, which would is
considered a break in period. This breaking period generates high heat
conditions in the axle which was found to degrade the synthetic beyond the
point of mineral lube. After the 500 mile lube change, the axle is broke
in and does not generate the high heat. If you use GM synthetic after the
break-in period, it can be considered "lube for life", although some say to
change it at 40,000 - 50,000 mile intervals."

"If you don't tow, don't worry about the oil change as it a waste of money."

"If your truck is pre 1998 or the old style, it comes with mineral lube. IN
that case you need to stay with mineral because the seals are designed to
work with it. If you changed from mineral to synthetic you'll cause your
seals to swell and eventually leak. It's ok to go from synthetic to
mineral because you won't cause swelling. many of your stop leak
chemicals effect seals in this manner, resulting in a short term fix but a
long term problem."