Hello everybody, my sister bought a VW LUPO 1.4 TDI. Today she showed me the car and in the trunk I discovered a funny tank, she said she didn’t see it, because the rear seats were folded and the summer tires were there. Now I wanted to put the summer tires on it and notice this funny tank. (my sister hasn’t filled up yet) Can someone tell me what it is (see pictures) and if I should fill up there? Thank you very much.