[rotorwrench]

Altitude has a big effect on vapor pressure. This is evident in the make up of avgas. The addition of tetra ethyl lead is only part of it. The blend to control vapor pressure is the other part. 100LL is expensive for a good reason. Mogas can leave a lot to be desired and especially at higher elevations. Butane is used in winter fuel to increase volatility for starting. Even the ethanol level can change the Reid Vapor Pressure somewhat. The OP may have to check other sources of fuel to find better functionality.