I had my gearbox and diff oil changed as soon as I brought my zed last July. Also added Molyslip into the gearbox. I would say there was slight improvement.
Not sure how often they should be replaced but if they haven't been changed, definitely worth changing. Did you go through your service history in details? Does the full service includes gearbox and diff oil change.
Ewen at Clarks Motorsports can sort you out on this. I got mine from him, service was excellent! Highly recommended