There is no filler on gearboxes as such, it won't need topped up unless you have lost oil out of it,if you have it will be a garage job anyway.The oil goes in a plug usually on the front of the gearbox about half way up,it's not part of routine maintenance and shouldn't need touched for the life of the car (unless you have a puddle on the floor)forget about it.
Rich
April 2011
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