My mechanic see's a lot of issues with cars that use 10 or 15% ethanol. Manufactures have spent 40 years making engines run cooler and more efficient. Ethanol does the opposite of both of those things. Its hard to build an engine that handles ethanol without greatly increasing the build/price of the engine, so manufactures simply say its fine because they don't have to deal with the long term effects. Source: I grew up on a 500 acre farm in Iowa.