One of the reasons for the poor performance of Italian aircraft engines was the use of lower grade gasoline. The engines were designed for inferior fuel because Italy was dependent on imports. IMO Mussolini's greatest failure was the lack of effort to develop the Libyan oilfields. The oil was known about before 1920.
The Axis in Europe had to make do with 87 octane gas for aircraft. The US produced 100 octane aircraft gas and shared it with the British; both got the extra performance from it. The DB601/605 engines would've been designed even better if aviation gas was available to the Germans.
I don't knw what the Russians used. My guess is lower grade.
Here's an article on 100 octane gas. Developed by a Frenchman but used by the US. It says the Germans had 89 octane gas and the Japanese 87 octane fuel.
https://sofrep.com/news/filler-up-how-high-octane-gasoline-saved-untold-allied-pilots-during-wwii/
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