Remember that article in the Wall Street Journal I mentioned earlier, the one that quotes Chuck Fort from BOAT/US? He said, ". . . ethanol is a powerful solvent." After thinking about this for awhile, and after reading an NMMA (National Marine Manufacturers Association) report, I came to the realization that his statement is a little misleading. In fact, ethanol isn't any more of a solvent than grape juice, or, for that matter, gasoline.
The problem isn’t that ethanol is a solvent, it's that it is incompatible with butyl rubber (e.g. O-rings and hoses) and a certain chemical known as di-iso octyl phtalate, a plastisizer used in fiberglass resin. It also scrubs the protective oxide layer off of aluminum and is electrically conductive. In phase with gasoline (meaning all together as one system), and in blends of 10% or less, most experts will say fegetaboutit. But as I said before, ethanol has this ugly habit whereby it sticks to water more than gasoline. In other words, ethanol will hook up with the condensation in your fuel tank over the gasoline there every time. When it does this, it sinks and concentrates at the bottom of the tank.
So what? So you might have ethanol and water at the bottom of the tank? Big deal. Well, in a fiberglass tank, the ethanol invades the pores of the tank, releasing some of the phtalates. These enter the combustion process and end up forming heavy black deposits on intake valves. That's the theory, anyway, based on analysis of Connecticut valve crud, which shows up as di-iso octyl phtalate.
Aluminum is another story. An oxide layer protects aluminum. No oxide layer. No corrosion protection. Ethanol and water are electrically conductive, so you have a galvanic issue inside the tank to worry about.
Hope this helps.