I would imagine the fuel was progressively higher-octane aviation gas made possible by an unlimited supply of American oil.
Unlike the Luftwaffe which was limited to 84-87 octane from captured oilfields in the East, and liquid fuel derived from coal.
Don’t know about the island.
Reading WW2 newspapers, I’m under the impression that we were providing 100 octane gas to England, both for our planes and theirs.