This thread has wandered into discussing the British wartime policy of water supply and that of petroleum, oil and lubricants which directed how the various containers were distributed and used. For fuel you had various bulk and medium size containers, so in the case of the rectangular 4 gallon tin (flimsy), this was considered a non-returnable bulk storage item. On the other hand the 4½ gallon Jerrican was considered at the time as a medium sized returnable item. I have reference to non-returnable fuel containers being used in the UK prior to Normandy and direction that they were not to be used as water containers afterwards.
Wartime terminology refers to the returnable containers, the items most likely found on vehicles, as British - Jerricans, US - Amerricans, and British - 2 Gallon Cans.
Also 4½ gallons of fuel weighs 27lbs or 12kg. The same amount of water is 32lbs or 14.5kg.