The gearbox is assembled in a clean room because the tollerances in the pump & motor are so fine it is not funny.
Thus opening the box to put new oil will do more damage than good.
I take them into the bathroom and assemble them in a wet shower cubicle in order to get a dust free enviroment.
The box is sealed and best left that way.
Boxes designed to be opened have very fine oil filters on them mainly to remove impurities introduced when topping up the oil.