Don't want to get caught up in the argument, but considering you're starting off with maybe a 1000hr engine life, service every 30 say, using that stuff means you might get 750hrs and should service every 10. It doesn't mean your engine fails after a few flights or anything, unless the engine itself can't really cope putting out too much power (which is really just a question of fuel/air combustion limitations for an inline engine of a given construction).
Using 1c Maddox as a source on this point, the AM and ASM engines introduced at the start of 44 (basically just the 605A/AS but modified for high octane fuel and MW-50 use) had a reputedly terrible engine life and could fail during a mission with overuse of the MW-50 system (even within operating guidelines). The D series engines and a virtually interchangeable redevelopment of the AS engine during 1945 were a much different story however, and the additional cooling capacity of the new airframe helped give this model no real faults to speak of, which weren't quirks plaguing the series (it is said the late 109 did not tolerate fools kindly, and killed a few on the runway).
At any rate keep in mind the Gustav remained in combat service with some air forces (Spain, Czechoslovakia) in one form or another well into the 1950's. Their competitors were generally the P-51D Mustang and Mark XIV Spit also exported widely, so put simply they can't have been too bad according to impassive history.
