What seems a bit interesting is that after GM went to syn rear axle lubes, they continue to recommend changing it every so often in the 2500HD pickups, which apparently is necessary for best longevity.
All of the prior-gen rear axles (i.e., but not limited to, Chrysler 8.75", Chry 9.25", Ford 9", Chevy 10-bolt and 12-bolt) all lasted just fine with the normal 90W axle grease in them. Just getting things topped-off every so often during an oil change. HD Trailer or other severe use service schedule probably did recommend changes at some mileage interval, but the normal daily-use cars almost never got any rear axle lube changes for well past 100K miles, if then. Of course, suctioning it out and putting in fresh lube is probably good, too, especially if the car is used in colder climates and is driven short distances each time it goes out. Just as with motor oil, the oil needs to get hot enough for a long enough time to cook-out the accumulated moisture/condensate in the rear axle assy and its lube . . . for optimal results . . . regardless of whether it's "dino" or "syn".
Just as with motor oil, heavier gear oil absorbs more horsepower than the normal (85W-90) viscosity will.
Just some observations,
CBODY67