Fuel Types     
The most common automobile fuels are now designated with an E moniker. The designation is indicative of ethanol content in fuel. It is a ‘by volume’ measurement. Most common automobile fuels (over 90% of the US market) now contain ethanol.
E10: up to 10% ethanol in fuel. All cars can use this fuel type and it is the most common gasoline blend sold today.
E15: up to 15% ethanol in fuel. Only vehicles made after 2001 can use this type but in reality many engine manufacturers do not support a warranty claim if gas with ethanol levels greater than 15% is used.
E85: 85% ethanol fuel, used by Flex-Fuel vehicles only.
E0: no ethanol content, also known as pure (‘neat’) petroleum fuel (but now can be difficult to source).
Besides the E designation, an automobile fuel is classified according to octane level. Simply put this is a measure of the ability for a fuel to withstand pre-ignition (also known as pinging – more below). The higher the number the better the engine performance and better the engine life, all things held equal. Today’s fuels use ethanol as an octane booster. Pure ethanol has an octane rating of 113 – 114.
What Happens to Fuel When it is Stored? 
The single most important issue for stored gasoline is phase separation. The alcohol (as ethanol) absorbs water rather easily (alcohol being a hydrophilic substance). Free water is available through direct air contact (humidity), storage tank leakage or through temperature change and condensation of absorbed water in a fuel mixture. At a critical point (Saturation point) the alcohol and water will settle to the bottom of any tank (large or small) due to density difference. The gasoline left above (in the tank) will have a lower octane rating. The bottom material will not ignite in a modern combustion engine. Some refer to this deposit as “gum”.
Varnish is the residue of old fuel left on engine surfaces. Hot engine shutdowns can lead to ‘heat soak’ whereby rapid fuel evaporation on the tips of the injectors leaves a residual varnish that can cause future engine system clogging.
Lowered Octane Level
The gasoline blends of today are given an octane boost through the added ethanol (previously performed by a chemical additive called MTBE). As a consequence of water-induced phase separation, fuel blends can end up with lower octane than specification and become unusable (and the fuel may have to be disposed of with clear cost consequences).
How long does it take for Phase Separation to occur?  
Phase separation of a water-ethanol mixture from gasoline will occur for a variety of factors and at differing rates. The critical measure is the saturation point (0.05% H2O at 60 oF) as it is this concentration whereby the water-ethanol mixture separates from the gasoline and settles to its respective container bottom.
Clearly any direct contact with water will accelerate the rate at which phase separation will occur. Tank condensation, contaminated fuel containers, vent seal leaks and container cracking are all direct means of moisture addition.
Temperature greatly affects the phase separation point. For example, a drop in ambient temperature from 60 oF to 28 oF will reduce the water carrying ability of an E10 fuel mixture by approximately 33%.
Most common gasoline-ethanol blends will start to breakdown after 30 days. Fuel does go stale just like milk and bread. The oxidation ability of ethanol in the fuel accelerates the ‘going stale’ process. In addition, over time, as a gasoline source is subjected to multi transfers and ‘re-fills’, moisture will inevitably enter the mixture (mostly through air contact). Operators and vehicle owners need to be aware of this fact and react accordingly.
What Damage can be caused by Expired and Old Fuel? 
Fuel that has phase separated is corrosive, namely the high ethanol concentrate at the bottom of an affected storage vessel. This can cause damage to non-metallic and rubber components (gaskets, hoses, O-rings, etc.) in storage tanks, ancillary dispensing equipment and ultimately the automobile fuel system. It is important to understand when phase separation has occurred to prevent this material from contacting areas sensitive to corrosion.
Pinging in an engine (that knocking sound heard when a vehicle accelerates) is a result of poor combustion within the engine chamber and can be caused by a variety of factors. Aged fuel that has a low octane content can cause pinging as early combustion occurs. The fuel mixture ignites prior to spark plug firing leasing to dual flame front collisions (to be technical).
- Clogged fuel system components
The gum of partial to full phase separated fuel can clog fuel lines and injectors, coat engine components such as valves and carburetors, lead to sensor issues and plugged catalytic converters.
Any varnish particulate deposits can lead to plugged injectors, fuel spray delivery problems and engine issues ranging from delayed acceleration and stalling to poor fuel economy. In extreme cases valve stems can stick from varnish deposits causing extensive engine damage.
How to Prepare Fuel for Storage? 
- Fuel tank
The ideal fuel preparation for storage of a vehicle is to empty the fuel tank. This may be a challenge for some operators (on a few levels) and could lead to future varnishing issues (and condensation) if not done correctly. As a minimum it would be best to fill a tank (to 95% level) to prevent any condensation potential. It is important to remember that once a fuel becomes stale and phase separation occurs, no amount of re-mixing will restore the fuel.
- Ideal environment for fuel storage
Temperature has a significant impact of fuel stability. High temperature (along with high humidity) is the main situation to avoid. Storing fuel (and the vehicles in which it rests) in a cool, dry environment is preferred. A stable temperature below 80 oF is a good guideline to follow.
- Best fuels to use for vehicle storage
The use of conventional E0 gasoline is the best fuel to use when storing a vehicle (with residual fuel). While not readily available, it can still be purchased in some states and locales. Use online forums such as GasBuddy.com and REALGAS.com to locate but there is no guarantee that it is available in all areas. The ethanol content is not controlled at the refinery but at the distributor level.
- Fuel treatment
One means to keep the residual fuel from breaking down is to add a commercially available fuel (ethanol) stabilizer to the fuel tank before putting the vehicle into storage. The product is best added to a full fresh tank and it is recommended to run the respective engine for 5 minutes to ensure proper mixing.
1. Gold Eagle Co. http://www.goldeagle.com/engine_care/411onethanol.aspx
2. Gold Eagle Co. Changes in Gasoline IV http://www.goldeagle.com/UserFiles/file/Ethanol%20411/Changes_in_Gasoline_Manual_IV_Updated_Logo.pdf
3. U.S. Energy Administration http://www.eia.gov/tools/faqs/faq.cfm?id=27&t=10
4. U.S. Department of Energy http://www.fueleconomy.gov/feg/ethanol.shtm
5. National Petroleum Equipment CO. http://nationalpetroleum.net/Ethanol-Water-Phase-Separation-facts.pdf
7. Fuel-Testers http://www.fuel-testers.com/gasoline_octane_and_ethanol_E10.html
8. Fuel-Testers, Ethanol Laws State by State Guide http://www.fuel-testers.com/state_guide_ethanol_laws.html
9. Fuel-testers, Precautions/Tips for Use of E10 Gas http://www.fuel-testers.com/ethanol_engine_precautions.html
10. Pure-gas.org http://pure-gas.org/
13. Pinging http://repairpal.com/what-is-engine-pinging
14. General comments on Ethanol in Fuel http://www.williammaloney.com/Misc/EthanolProsConsDrawbacks.htm