The water that gets trapped in the gas causes both direct corrosion and also acid to from that causes further damage. So its not really the GAS itself but it is the largest contributor.
'Pure' gas would cause varnish if it dried out. Ethanol gas really doesn't have a varnish issue as the alcohol removes it.. but the water is a big problem.
If your carb is in good shape and still has a protective zinc-cad or other coating it's safe. But the carbs where the coating is gone (its not the shiny zinc, but the dull blue-gray color) can get damaged by the ethanol fuel.
Plus every state may or may not have all the pumps marked. A pump can be labeled (in PA) as 10% ethanol but it could pump 100% gas... some states have no pump marking requirements... so in all reality you may or may not be even getting 10% ethanol gas as not car REQUIRES it. On top of that you could be getting much more than 10% ethanol... there is no test at the pump to check it and (again, in PA) there is no certified sticker that says 'Yep, 10% corn'. Weights and measures only guarantees a gallon is a gallon.... but a gallon of what?
