I don't want to muddy the waters any more than they are already regarding the 10% ethanol question. However when ethanol fuel first appeared on the market I decided to change my aircrafts fuel lines. Problem what to buy? I soon found out some sellers were selling hose which was unsuitable for ethanol under the guise of being suitable for unleaded. So beware. I wondered what 5% ethanol looked like, so I took several samples from the major suppliers and tested these samples. Lo and behold no two were the same one showed as little as 1% ethanol whilst others showed varying amounts one sample showed as much as 9%. I was lead to believe, that tankers leaving the refinery had 5% ethanol added, presumably to mix whilst slopping about in the tanker. I am unsure of the metering and mixing process, but I have no reason to doubt the accuracy of my testing. Incidentally the 1% and the 9% samples were taken from the same supplier. It appears we are stuck with 10% at the moment, but I forecast 15% and beyond in the future.
Total fuels produce an ethanol free fuel for aviation use called UL.90 not all airports carry it but some of the smaller ones might, if you are lucky enough to have a small flying club type field in your vicinity you could always ask.
