nowadays it's not uncommon to see e10, e15 and even e85 for flex

the real problem is the potential damage to engine internal for water content in ethanol. there is an article credible here auto.howstuffworks.com/fuel-efficiency/alternative-fuels/ethanol-damage-engine.htm

if you can avoid using gas with ethanol above 10%, preferrablly non ethanol, you would have less chance of such damage and getting better gas mileage.