How did they get improved performance out of the fuel then?
Higher oct fuel + low-ish CR = greater boost = more power. Works for all countries' supercharged engines.
The Mikulin AM-42 with CR 5.5:1,using boost of 2 ata for 2000 HP, on Soviet 96 oct fuel, no water-alcohol injection, no intercooler.
In the same time, DB 605L has the CR of 8.5:1, and must use MW50 even when using boost above 1.4 ata + C3 fuel that was 100/130+ grade in 1945.
Why weren't they used more then in German designs? AFAIK the Jumo 211J was the first that had one.
Again - who knows? We know that Jumo 213E and DB 603LA used intercoolers (and could use B4 fuel). The 213F, 603L and 605L did not, the last two needed C3 fuel and MW50 to operate at hi-power settings, 213F used B4 + MW50.