Is filled by a 2-cent coin-sized hole. I can recommend here a medium sized hopper jammed exactly pure. Stand firm on the floor, you can easily fill out nem 5-liter canister 1189er LM. Screw it (is as large as about a WaMa port-clamp) and into the tank. Is also strong enough to push the cap of the filler neck on side. Also angle and length of the proboscis fit, so you can squeeze everything out of the bottle to get out a few drops (here Astra H GTC, filler very deep-pocketed).
Thoroughly recommended, full 5 stars!