Our friends at ValvTect offer an explanation of how the addition of ethanol undermines octane and fuel efficiency:
There are three potential causes for “octane loss” and unfortunately none of them are linear.
First: Ethanol separation.  In regular E-10 gasoline, ethanol accounts for about 3 octane numbers.  Should the gasoline phase separate the gasoline on top of the water/alcohol mix will be about 84 octane.  Phase separation is not a linear function.  The fuel is either phase separated or it isn’t.  As long as there is no phase separation there won’t be any octane loss.  That is why ValvTect Marine Gasoline (VMGA) and ValvTect Ethanol Gasoline Additive (VEGA) are formulated to inhibit phase separation.
Fuel Dock
Second:  Oxidation.  Gasoline is susceptible to oxidation.  That is after all why we use it as fuel.  In the engine gasoline is rapidly oxidized; it combines with oxygen and is converted to carbon dioxide and water.  In the gas tank, on the other hand, this oxidation occurs as well; but very, very slowly and the reaction does not go completely to carbon dioxide and water.  In the fuel tank the fuel only partially oxidizes and forms ethers, aldehydes, esters and ketones.  Left unchecked these oxidation products proceed to form the gum and lacquers that make a mess of fuel systems.  They also don’t burn as efficiently as the initial gasoline and can lead to octane loss.  This process is not linear.  It is logarithmic.  Things go along fine to a point then the oxidation takes off.  Think of a hockey stick.  Oxidation can be inhibited with antioxidants (stabilizers).  Stabilizers are added to the fuel at the refinery to slow oxidation and get the fuel through the system.  In typical automotive use the fuel is burned up before oxidation can cause a problem.  When fuel sits around for a while additional stabilizers are required.  That is why ValvTect Marine Gasoline and ValvTect Ethanol Gasoline Additive have additional antioxidants.  ValvTect uses the same antioxidants as used by the major refineries.  The additional antioxidant in VMGA and VEGA will keep the fuel fresh, prevent octane loss from oxidation, for at least a year.
Third: Volatility.  Gasoline is a blend of a number of different hydrocarbon molecules.  These molecules contain between 4 and 12 carbon atoms.  Some of the octane rating is attributed to the short chain molecules like butane (C-4).  The problem with the short chain molecules, is they tend to evaporate in an open, vented, system such as a fuel tank.  The evaporation is temperature dependent, much less occurs in the winter and more in the summer.  Gasoline volatility might cause about 1 number, maybe 2 numbers in an extreme case, of octane loss.  Volatility is very difficult to predict, but provided the other two mechanisms are addressed, volatility has not been a problems in typical winter storage conditions.
Regards,  Mark Hildebrandt
ValvTect Petroleum Products