AFAIU the problem wasn't that the 601N couldn't efficiently combust C3 per se, but rather that minute amounts of fuel found their way past the rings into the crankcase (as happens with all engines to some extent), and because of the high boiling point it didn't evaporate away, and over some time as the concentration of fuel in the oil increased the lubrication qualities decreased until either the bearings were damaged or even the engine seized outright.
The composition of C3 was changed over time as well to shift the distillation curve downwards, C3 in 1940 wasn't the same thing as C3 in 1944. But by then the 601N had long been withdrawn from service.