I would agree it's an oil can but it's different from the Vickers cans I've seen. The screw top on a Vickers can has a brush attached. The screw top and brush are exactly the same as the ones screwed into the handles on the machinegun
Nice know what this tank! was for,I knew it didnt hold much as size was in listing,but I do know a guy who has a old bleach bottle strapped to his 65 Triumph carrier which contains a litre of petrol just in case he gets caught out low on petrol as he says it will get him 10 miles if he takes it easy which should be enough to find a petrol station also its 10 miles less to walk...Dave