Related to this discussion, I found on a Rotax engine website where they say that leaded fuel such as 100 LL aviation fuel can damage valve seats. Here is the link and the text: http://www.rotaxservice.com/rotax_tips/rotax_feed4.htm Aviation Fuels It is possible but not recommended to use 100LL AVGAS, since the the lead content is like cholesterol to your engine: it will accelerate wear on the valve seats, create deposits in the combustion chamber and sediments in the lubrication system and gearbox. Increased maintenance is necessary to compensate. Unlike "conventional" aircraft engines, lead is absolutely not essential to the proper lubrication and operation of a Rotax 4-stroke aircraft engine. The increased octane rating also has no marked advantage for the operation of your engine.