I worked on a 160hp SUperCub in Alaska that belonged to an outlaw trapper guy. He never got annuals, no pilot license and he has over 11,000 flying in a Alaska. He insisted on using unleade car gas in his 160 high compression engine so I had a fuel blender at a local refinery help me and we determined that you only have to add 10% 100 LL to unleaded car gas and it raises the anti know index over 100. Before we did that , the trapper burned a couple of pistons in the summer on a hot day (80) low level full throttle....after we came up with the mix , no more problems.... TEL was used because it only takes a very small amount to raise the anti know indext right up there... So I am guessing in a Rotax little 100 LL wont hurt and will likely help prevent detonation or pings....The problem with a full diet of 100 LL is it will coat the inside of the combustion chamber and raise the effective compression ratio. Then if you put some cheap car gas in the same engine later, you might get into detonation area,especially at low altitude and full throttle on a hot day..
Car gas has butane and other aeromatic fuels added to raise the antiknow index but it these things that cause problems with rubber hoses and gaskets.
Alcohol is a another problem but pure alcohol has an antiknok index of 100 so it helps raise the antiknow index number.
exthane, methane, propane , butane , pentane, ?,?,iso octane.... Iso octane has a knock index of 100 and that is what was historically used in a test engine and then the ratio was compared to regular car gas as to what pressure it could take before pinging.....Alkand parafin series of hydrocarbons are what is in the fuel...
SO in summary, adding 10% avgas 100 will raise the anti knock index and provide some protection against detonation. Avgas is stable and will store for several years. I have used some gas that it over 5 yrs old....The real problem is the EPA was sued by friend of the earth and so there is a deadline to remove the lead from the fuel....so far, there have been limited results. The test engine is the Lycoming TSIO 540 J2BD engine with is turbo supercharged and makes 350 hp at 41" MAP so the thinking is if an unleaded avgas can be made to run in this engine, it will run in the rest because this engine has a fairly narrow detonation threshold.I was lucky to have a petroleum engineer who designed and built the refinery at North Pole, Alaska and another flyer who was a fuel blender at the refinery so they taught me a lot about fuels. The petroleum companies will tell you nothing because of potential liability...