E5 would have been a better choice by Congress but the Corn lobby pushed thru E10 or 10% alcohol mix with gasoline which is OK with seals and elastomers at room temperature (~72°F). Unfortunately a carb on top of an engine, fuel pump on the side, or a vehicle sitting in the sun gets a mite bit warmer so the E10 "rots" out the elastomer parts. The hotter the fuel the faster it "rots" them out. It is not actually "rot" but the term is as good as others. The alcohol swells the surface and produces fine cracks from the expansion stress. The surface eventually crumbles off like dust or fine cube shaped particles. Different elastomers (rubbers) behave similarly bit the various types have different sensitivities to alcohol. Some like Viton are better than others.

The alcohol absorbs moisture from the air but it also leaves it behind when the temp changes or the alcohol dissipates. This forms liquid water drops in the fuel system. Liquid water will clog passages and cause corrosion on dissimilar metal parts in the fuel system (usually a white powder on aluminum, "rust" brown on steel). Since sometime in the 80's fuel systems have had a coating inside the fuel tanks and lines to resist this water corrosion. Older fuel systems and carbs don't have this protection.

Running E10 fuel thru the system rapidly, -like with an every day commute works OK. The fuel is cycled thru rapidly and does not have time to let the water accumulate. A vehicle or small engine that is only operated occasionally is a bad place to use E10.