I use no-Ethanol 92 car gas from my hangar, but if x/c the airports almost always sell only 100LL.
So, a mixture of the two is inevitable if you travel in your gyro.
If you use 100LL in any amount, you should also be using at least a semi-synthetic motor oil, if not a mineral-based oil.
Pure synthetic oil will not capture the lead deposits of 100LL, and will actually sludge up.
That said, I see no value in purposely mixing them, and some possible detriment to doing so.
Thanks for the cited tips, Kolibri. I assume anything good for a 4 stroke Rotax is good for the Soobee too, I may have been given some bum information by well-intentioned experts. Getting quite comfortable now, flying about an hour each day, mostly S&G. Will start practicing aerial maneuvers soon, and having a blast. That cabin heater works pretty well.
The lead in the aviation fuel will foul your oxygen sensors used by the computer to inject the proper amount of fuel. I have heard of people using TCP to bind with the lead and prevent this from happening, but I don't know if it really works. My subaru has fouled a sensor using avgas. It runs really rich and gas consumption is horrible. It gets worse the higher you go.