Hello. I needed to top up my gearbox oil as it's become stiff to get into gears. When I went under the car I can see the fill plug has been stripped (highlighted in red) and a 8mm bit will just wiggle around in it. It's also very hard to get to and can't really get straight tools in there.

I've done some research and it seems like the oil can be topped up through the reverse switch highligh in green. Does anyone if this is true and what size socket will go onto it as the bolt is bigger than 21mm socket I have?

It's just a hole into the gearbox, so yes you can put oil in there.  But it's at the wrong height, so you won't be able to check the level accurately.

If the gearbox feels stiff that does not necessarily mean that the oil level of the gearbox is low.

These gearboxes are considered to be filled for life and are completely sealed. The oil level can only drop in case of a leak.


The oil level must be 5 mm below the lower edge of the filler plug. The only way to check the oil level is by removing the filler plug. Since too much oil is just as bad as too little oil, I strongly suggest attempting to remove the filler plug

If the internal hexagon of the filler plug is rounded the filler plug should be replaced. The rounded plug can usually be removed easily by welding a nut onto the plug. Once the nut is properly welded onto the plug the plug can easily be removed using a suitable spanner.

Another proven method is loosening the filler plug using a small chisel and a hammer. 

 

you can try a hex-bolt extractor.