best fuel option - 100LL, 10% Eth, or Mix

I mix 91 octane (no ETH) car gas with 100 LL with decalin. 50/50 mix.

Ernest, in the Rotax MML, they list additional mx or more frequent intervals if more than 30% leaded fuel is used.  Curious why you use a 50/50 mix as you don't get the benefit of unleaded fuel at 50/50 and have an Octane rating higher than required?

hi Tom

The idea that Avgas is the same in octane measurement method is not correct.  Avgas is on a MON scale, auto fuels in the USA and Canada use AKI, an average of RON + MON / 2 method.  Because of this you can not use the formula you propose.  50/50 mix is far easier and "tribal knowledge" has shown it works well with the high compression Rotax engines.  

Under no circumstances should you use any auto fuel less than 91 AKI on a 912 ULS.  That engine has a nearly 11 to 1 compression ratio and less than 91 can lead to detonation inside the engine. 

Rotax has tested Ethanol blended Mogas, up to 10% is fine for the engine as long as you do not store the fuel for long periods of time.  Ethanol blended fuels can have issues after only 6 months from the date of its manufacture.  In practical terms cut that in half to allow a margin for transportation, storage time, before you may actually use it.  Fuels without Ethanol double that time.  Avgas in a sealed metal container are good for 1 year.  All fuels when open (vented) can evaporate and they start to change.  Best advice is to buy it and use it as soon as possible.  Don't store fuels for long periods of time.  

Cheers