Differentials of today are subjected to "Severe" duty service and encounter more stress and heat than was seen only a few years ago. Most modern gear oils are faced with the challenge of providing adequite wear protection during severe service operating conditions, while also providing maximum fuel efficiency.
Most of the wear that occurs in differentials take place during the break-in process!
Amnsoil Inc. did a test on 4 seperate vehicles, and demonstrated the importance of changing the "factory fill" gear lube within the first few thousand miles. Oil analysis results revealed most of the wear in vehicle #1 occured over the first 6,869 miles on the vehicle as well as the gear oil with 493 ppm Iron. Vehicle #2 had 16,766 miles on the vehicle as well as the gear oil and 542 ppm Iron with the factory fill oil.
Vehicle #3 had 50,994 miles on the vehicle as well as the gear oil with 608 ppm Iron on the factory fill oil. Vehicle #4 had 146,764 miles on the vehicle, and 18,101 miles on the gear oil and 83 ppm Iron, but was filed with Amsoil "Severe Gear" 75W-90 gear oil.
All of the vehicles gear lube's were still in the Viscosity range for their respective oil grades, and each of the 4 vehicles were factory filled with GM's specified lubes.
The bottom line is this:
Change out the factory fill gear lube within the first 500 to 3,000 miles and install a high quality synthetic gear lube, then change at pre-determined intervals depending on the way you subject your truck too.
For those that use the Amsoil brand gear lube, Amsoil's recommendation on the Severe Gear 75W-90 gear lube is 50,000 miles for "Severe" service applications, and 100,000 miles for "normal" service applications.
wayne
amsoilman
Attention: TDR Forum Junkies 
Also oil level in new MAG HYTEC cover has higher oil level marks than stock by about 1 1/2 ins. if I fill it to mag-hytec recomended levels will I have any trouble such as axel seals leaking.
