In the German BMW R12GS forum I read a lot about drilling the tank nozzle, allowing fuel top-up topping to be easier and quicker. The guys who drilled obviously got 2 liters more into the tank. That's quite some difference, but I am not in the mood to dismantle everything.
Then looked into the tank and saw a rubber grommet
Took it off, its easy. Poke the fingers in and lift it up. Refueling is much easier without. I think the rubber grommet is there to prevent people using leaded petrol?