The parts manual is wrong! The part listings under the MC18 fuel tank diagram are for the MC16 tank. I think this isn't the only error of its kind in the manual, so watch out when ordering parts.

There seem to be two critical differences in the tanks, which make cross-compatibility difficult. One is the shape of the rubber mounts that each end of the tank sits on. The other I'm not sure about, but I think the MC18 tank is 1-2cm longer. How do I know this? I have an MC18 tank on my MC16. It came with the bike, and it looks like someone has welded a piece on the front of the tank to fit the MC16 frame, and they welded a new mounting point (on the subframe) about 1-2cm back from the original. I've since put on an MC18 subframe I had spare, and the tank now aligns perfectly with the original threaded mounting point.

In summary, I would buy an MC16 tank.