RE: How often should diff oil be replaced?
My Hyper 7 is more than 2 years old raced off and on road and i changed the diff oils 3-4 times...gears look as new!
As long as there is oil in there and it's not THAT old it will be fine.
BUT you have to check them often.
On the center diff it's easy to see if it's leaking...on the rear and front though you have to check them by opening the gear case.