Have Castrol Syntrax Universal Plus 75W-90 in my 75.
Castrol, Shell and probably a few other manufacturers can carry out an analysis of your oil. Probably best way to determine if your oil needs replacing, if your using the correct oil and likely condition of bearings, gears and synchros etc.
The analysis should include a comprehensive viscosity test where the degree of shearing of the polymer molecules is determined. If the viscosity has reduced because of excessive shearing then definitely time to change oil and probably oil type.