Petrol tank, used with traces of petrol....hmmm...maybe the airlines wont think too kindly of that idea.

Depends where you may, may be easier to get a new tank and just bring it back, brand new and unused. May have to freight it seperately to you and your checkin luggage too, come back EMS or UPS/Fedex as a new unused tank, cause even they would consider a used tank as "dangerous goods".