newmercman:
It all depends on your operation, the best way to tell is to drain a bit and send it off for analysis,Oil does not go off, it gets overwhelmed by contaminants and in a closed system such as a gearbox or differential, contaminants mean wear metals.
Analysis is the ONLY reliable method of determining oil change intervals.
Most do not have a closed system anymore eg one way breather valve.A top hat seal.
Most have an open vent system that allows expansion but also allows water ingress as the axle cools sucking crap into the axle banjo.
Also causes hub seal failure due to breather blockage and pressurising the axle while in operation.Once upon a time you pulled a diff plug and it had a vacuum behind it now the oil is at atmosphere and black as your hat after 100.000 km.
Used to change oil on Rockwells after 500,000 km and it was as clear as honey.
But of course that dont sell axles does it.
Gearboxes had the same idea but again it does not sell gearboxes does it!