Phase separation of gasohol results in a layer of gasoline atop a solution of ethanol and water. You don't want this in your tank!
Author and chemist Benjamin Kellogg has an article in the Fall 2011 issue of Army Motors with the results of his experiments with fuel stabilizers.
E10 gasohol absorbs and retains water, the actual water amount varying with temperature.
At 70°, E10 can contain as much as 0.5 ml of water per 100 ml of E10.
Once the water content exceeds this limit, "phase separation" will occur, where the denser ethanol and water components settle to the bottom of the fuel tank as the less-dense gasoline components rise to the top, an essentially irreversible process.
Fuel additives such as Sta-Bil contain napthenic oil, hydroethylated aminoethylamide and petroleum naptha. They
decrease the probability that phase separation will occur in the fuel tank of stored vehicles.
E10 gas without additives undergoes phase separation with the addition of 0.30 ml water. Addition of either Fuel Protection or Fuel Stabilization formulas delayed phase separation until the addition of 0.50 and 0.49 ml of water, respectively, but the addition of *both* Fuel Protection *and* Fuel Stabilization formulas to 50 ml E10 delayed phase separation until 0.69 ml water was added.
The reasons for this aren't clear; it might only be because the ingredients are doubled.
For more on the effects of E10 in vintage cars, see the Nov. 2011 issue of Car Collector Chronicles at http://www.scribd.com/people/view/7936333-dave
Michael Pollan's book "The Omnivore's Dilemma" explains the politics behind ethanol in gasoline, beginning with Nixon's Secretary of Agriculture Earl Butz turning farm price supports into corn subsidies, leading to corn overproduction and the need to find added uses for corn.
Always use top-tier fuels.
Check www.pure-gas.org .
Marine and aviation gasolines are alcohol-free. The law may prohibit direct fueling into cars.
Most stations in Michigan sell non-alcohol premium. Some add one can of SeaFoam to this and report no issues.
Change the oil, shut the fuel line off and run the carburetor dry before storing. Otherwise, top the tank with premium and regulary run the engine until hot through the storage months.
Don't store fuel for long durations in the tank if at all possible, as most start to degrade after six months unless stored in a plastic airtight jug, which will buy a bit more time.
Fuel Guard, Sta-Bil and Startron are good ethanol treatments. Sta-bil also offers a marine-engine formula, which offers twice the protection.