Can I blend 100LL with Mogas with ethanol?

Valter, I often run with a blend of 100LL and 92-octane mogas (10% ethanol) because it can be difficult to find mogas while away from home.  BUT, you're talking about significantly higher levels of ethanol than we see here, and WAY above the maximum Rotax recommends.  

Personally, I would just run 100LL, and add the Decalin fuel treatment additive to minimize the negative properties of the lead (sludge buildup, etc.).  I would increase the oil change frequency also, per the Rotax guidelines when running 100LL.  Finally, I would definitely use the Aeroshell Sport Plus 4 oil, again to minimize the evils of lead sludge...

Boa sorte! That's "good luck" in Portuguese, for those who don't know the language.  I was fortunate enough to live in Sao Luis, Fortaleza, and (briefly) in Rio de Janeiro as a kid.

You can blend, but if you want to bring your 25% mogas down to the recommended 10% it will have to be about a 2 to 1 ratio of avgas to mogas.

There have been many places in the world that deal with 17% - 25% ethanol. So long as the fuel tanks are okay with it it won't be an issue. If it was many of our autos would be sitting on the side of the road. Rotax posted only 10% because that's all they tested. Lockwood did a couple of teardowns with the 23% I believe it was after lots of hours and found nothing wrong. Using 100LL even with Decalin will still leave lead. It helps, but it isn't a cure all.

With fuels you can have some issues with high ethanol content.  first remember that the carb engines are fixed jetting, they do not have sensors to sort out the fuel burn by reading an O2 sensor in the exhaust.  Given this your fuel burn at that level, over 20% ethanol, will be very lean.  Rotax will never recommend more than 10% simply because of the issues with very high NOx emissions with that type of fuel.  The injected engine is no better since it is a map system and this was developed to run on Avgas or premium auto fuel of higher than 95RON.  (RON always lower than MON)

Avgas 100LL is about 100 MON value.  (ASTM reports normal at 102 MON) Mixing of the fuel is not always a simple math exercise but to lower the lead content it is a reasonable concept.  The issue with the ethanol is more complex.  Fuel producers use the ethanol to increase the anti-knock value of the base fuel.  this allows them the use of a much lower base fuel as the very high knock value of ethanol increases the batch significantly.  For North America we normally see 10% blends and these typically in premium fuel read about 91 to 93 AKI (RON + MON / 2 = AKI) the top 2 or 3 points of that are the effects of the 10% ethanol.  I can only guess by the same values that the auto fuel in Brazil at 25% would be more towards 5 or 6 points of the total.  

Detonation issues that may occur with such fuels are not tested, at least not by the factory.  Any use of such fuels would be on the owner and you would need to be careful at sea level and high-power settings.  (more ethanol might lean the engine burn at take-off power)  Perhaps if your flying out of say Brasilia where you have a higher elevation you can get away with this, be mindful of detonation at low altitude.  

As a note, I participated in the inspection on the effects of Brazilian fuel in Rotax many years ago and this is where the acceptance of the 10% came from.  It was done on a 912 ULS that ran over 100 hours on 22% ethonal over an 8 year period.  (inspections done in Americana)   At the time with the assistance of Ultramotores who were the iSC, indendent Service Center for Brazil at the time.  

Cheers

Off topic, but what the heck happened to the 100LL replacement fuel? Seems like we were almost there and then all of a suddenly, silence.