As has been said earlier in this thread, a lot of advancements in aviation fuel were made in the decade prior to WW2. There were a lot of parallel developments, but from an American perspective, Jimmy Doolittle did a lot of good work on this front. It's no coincidence that he became an executive of Shell Oil after the war.
It's also a big part of the reason why the US was able to rapidly develop "next generation" aircraft after entering the war. Any aircraft is built upon its powerplant, and the foundation for significant horsepower increases was "primed" by the advances made in the 1930s.
As a small example comparison of radial engines in the Pacific, the primary Japanese naval fighter, the A6M 'Zero', had its horsepower pushed from the mid 900s to over 1,100 (a ~16% increase) in roughly the same time frame as the 1,200hp F4F Wildcat was replaced by the 2,000 F6F Hellcat (a ~66% increase). The Double Wasp engine enabled the creation of the Hellcat, Corsair, and Thunderbolt - all potent fighters. When the Hellcat first entered combat, several Japanese pilots made the fatal error of mistaking it for the relatively under-powered Wildcat and had rude awakenings as a result.