While you are at it, install a magnetic drain plug.
Not sure if it's available in Norway, but here in the US, most auto parts stores sell a plastic pump that fits into the top of the bottle of gear oil, with a plastic hose and nozzle that will clip into the fill port. Each full stroke of the pump is one ounce. Makes a relatively easy way to fill the gearbox, once you've got the car up on stands (or a lift). And since you can measure how much goes in, you don't have to have the car level.
The stock fill plug (which is also where you check the level) is a square-headed pipe plug. It's kind of easy to round off with an open end wrench; and an ordinary box end wrench doesn't fit the square very well. My favorite tool for it is an 8-point 7/16" socket and breaker bar. Most likely you'll want to replace the plug, as they are generally somewhat mangled. Personally, I like to replace them with a "hex socket" pipe plug, which is hardened and turns with an Allen key (aka hex key).
Here's a (poor) photo of the fill plug, lifted from the article by Larry Young & Sam Clark on the VTR web site.