Ethanol is hygroscopic. As long as you burn the fuel in a reasonable amount of time it's not a problem, but when ethanol sits over time it collects water. Next

Ethanol is hygroscopic. As long as you burn the fuel in a reasonable amount of time it's not a problem, but when ethanol sits over time it collects water. Next time you run your engine, it pulls water into the engine. Now, if the engine runs long enough to fully warm up, this isn't the end of the world. Any water will turn to steam and be pushed out through the exhaust. However, if the engine only runs for a short period of time, water can sit inside of it and cause corrosion. Realistically this is only a problem if you repeatedly allow ethanol-containing fuel to sit for a long time, and also repeatedly run an ICE for very short periods of time. It is a bigger problem for boat motors, because they operate in a wet environment already. It is also a bigger problem for carbureted engines, where fuel sits in the carburetor bowl while the engine is not running. This allows hygroscopic fuels to corrode the inside of the carburetor while the engine sits. Ignoring ethanol, it is also true that gasoline breaks down over time. However, it typically takes many months before gasoline breaks down enough to worry. There are also additives that help stabilize gasoline for longer storage. TL;DR in a reasonably modern-ish car, it's probably fine. In small engines like lawn mowers, it is best to either use ethanol-free gasoline or to completely drain the fuel before storing the mower for the winter. In antique cars with carburetors, it is best to use ethanol-free gasoline.