Change oil in diff. and gearbox?

Going by my experiences over the years I would say diffs as tough as they come, its the gearboxes that need looking after, but I have noticed the newer motors take quite a bit less diff oil then before, I would guess that the changes date from mineral to syn.
Daf used to be every annual Y service then in 2006 they changed it to every Y3 for trans oils, gearbox oil was as black as they come from then onwards.