Everyone blames the ethanol. National lab research says the gasoline goes bad first, and that changes what actually helps.

Ethanol-free gas does not stop gasoline going stale

The most useful conclusion from the disagreement, and it saves money. NREL research found that the petroleum components of ethanol-blended gasoline become degraded and unfit for use long before the ethanol takes up enough water to phase separate, and Extension guidance agrees that ethanol absorbs atmospheric water extremely slowly and will probably stay in solution harmlessly. If staleness is the primary failure, then paying extra for ethanol-free fuel and leaving it in a mower for six months has not solved anything, because the volatile components still evaporate and still leave varnish. Ethanol-free fuel has real advantages for equipment with older rubber and plastic fuel systems, and it is not a preservative.

Premium gasoline is the most common wasted purchase here

Worth stating plainly because the instinct is so strong. Guidance is direct: high-octane premium offers no inherent benefit to a mower’s longevity unless it is specifically labeled ethanol-free, most small engines are optimized for 87 octane, and using 91 or 93 that still contains ethanol provides no extra protection or performance over standard unleaded. So somebody buying premium to be kind to their equipment is buying octane the engine cannot use, at a premium, in fuel that will still go stale. The label to look for is ethanol content, not the octane number.

If separation does happen, it is immediate and irreversible

This is why the mechanism is worth understanding even if it is not the commonest failure. Ethanol blends absorb water up to a threshold that depends on the blend and the temperature, cited by a chemist as around five hundred parts per million for E10, and once that is exceeded the reaction is sudden. A lab test describes it flatly: if phase separation does occur, the process is immediate, complete and irreversible. The instruction that follows is equally flat, that the fuel should be removed and properly disposed of, not remixed and not burned in engines, and that stabilizers cannot reverse it once it has happened. There is no shaking the can.

It separates at the bottom, which is where the engine drinks from

The detail that turns a chemistry fact into a mechanical problem. The described layering in a separated tank is gasoline on top, a milky layer of ethanol and water below it, and a small layer of pure water at the very bottom. The fuel pickup is at the bottom. So the engine is not being fed slightly worse fuel, it is being fed the corrosive layer first, and the described end result is a carburetor ruined with rust and corrosion. Worth knowing that those repairs are described as costly and not typically covered by warranty, which puts the cost of prevention in perspective.

The fuel left on top has lost octane, and that causes its own damage

A consequence almost no coverage mentions. When ethanol drops out of solution it carries octane away with it, so the gasoline floating on top is now a lower octane fuel than what you bought. STIHL is cited on the effect: the floating layer has a lower octane level than the original blend, which can result in unstable engine operation, power loss and major engine failures. So a separated tank has two distinct problems rather than one. The bottom layer is corrosive and will not burn, and the top layer is off-specification fuel that can damage an engine that does manage to run on it.

E15 is not a preference question

This is the one part of the subject with no disagreement. Small engines are not designed for blends above ten percent, and E15 is not approved for outdoor power equipment. Briggs & Stratton’s own guidance describes E15 attracting moisture that separates from the ethanol, creating a layer of water and alcohol at the bottom of the tank that can severely damage the engine and will void the warranty. The practical difficulty is that E15 now appears at ordinary pumps alongside regular, sometimes at a lower price, with the distinction carried on a label rather than in the layout. Read the pump rather than the price, especially when filling a can rather than a car.

Liquid water getting in is a different problem from humidity, and it may be the real one

A distinction the research camp emphasizes and that reframes storage. Extension guidance points at liquid water rather than atmospheric absorption, noting that a mower or tractor sitting under a tree or a tarp for months may experience fuel and water issues, and that fuel containers should be sealed. A tarp that condenses onto a machine, a perished filler cap gasket, or an old can that breathes will admit actual water, and that affects straight gasoline exactly as much as it affects E10. Which makes the fixes physical rather than chemical: a good cap seal, a sealed and vent-limited can rated for gasoline, cool shaded storage, and replacing old cans that allow air exchange.

Label the cans, and learn to see it coming

Two habits that cost nothing. On labeling, guidance is specific about the reason: clearly mark ethanol-free gas for small engines versus E10 for trucks or flex-fuel vehicles, because a household with three identical red cans has no way to tell them apart and the mistake goes into the machine that tolerates it least. On detection, check for water at the bottom of tanks, and note that clear fuel filters or separators with visible bowls let you spot separation before it reaches the engine. That is the difference between a diagnosis and a repair. And on the end-of-season question, both running the tank dry and storing it full with stabilizer are in common practice for different reasons: follow whichever your manual specifies rather than a general rule, since the sourcing here does not settle it.