As described in the link previously posted and subsequently dismissed by Dan as having nothing to do with the topic:
Basically you are pushing this intermolecular force back towards where it started before ethanol was added to the fuel. So E10 becomes maybe E8 for vapor pressure purposes? That might be a .1% effective VP change, but I don't know what the threshold is. Ethanol nudges the vapor pressure up far enough to cause driveability issues in hot conditions where the fuel reaches boiling point quicker and at a lower temperature. I have no way of measuring it. I only know the behavior seen in my own vehicle from what I suspected to be ethanol-related fuel problems this past summer.
Not making any dosage recommendations, but if that happened you probably used too much. I saw no telltale smokeage from my Chrysler's tailpipes with a qt/30 gal.
