I was told not to do the gearbox oil as it can do more damage than good... Diff oil, yep forgot about that - will add this to the list.
My mechanic told me that all the crap kinda holds all the stuff together inside the gearbox and TC. He has had no end of issues following oil changes... I dunno mine seems ok for now
in my experience, people that talk you out of doing logical things is only because they are not confident in doing them
I'm only going on what I've been told by a very experienced mechanic. How often should it be changed? Covered 93k now.