How are your gear shifts?
Gearbox oil isn't really critical - it is more to do with smoother shifts, especially in cold weather on start up, and to help prevent sloppy shifting much later in the boxes life.
Probably be perfectly OK to wait until your next routine service.
Gearbox oils, and rear axle oils MUST be drained when hot, ideally after a good spanking - so when you do get round to getting it done, state you want the oil drained the moment you leave it at the stealers.