What Kind of Gas Should You Run in a Lawn Mower?

You fill the mower from the same can you use for the car, the gas has been sitting in the garage since last fall, and now the engine sputters, floods, or won't turn over at all. Most people don't think about what goes into a lawn mower until it stops running. The gas you use — and how long it's been sitting — matters more for a small engine than it does for your car, because the fuel most stations sell today was blended for vehicles, not for the carburetor on a push mower. This post covers exactly what to put in the tank, what to keep out of it, and how to store fuel so your equipment starts on the first pull.

Can you use regular gas from the pump in a lawn mower?

Regular 87-octane unleaded gas from any station will run your mower, as long as it contains no more than 10% ethanol (labeled E10). That's the standard fuel most small engines are built for. Higher-octane premium buys you nothing in a mower — octane resists engine knock, and small engines run at low compression where knock isn't the issue.

The catch isn't the octane rating. It's the ethanol. Nearly all pump gas in the U.S. now contains ethanol, and the pump you use for the car is very likely E10. Your mower can handle E10, but ethanol brings a set of problems that show up faster in a small engine than in a vehicle — which is why what you buy is only half the decision. How you store it is the other half.

Why is ethanol bad for lawn mower engines?

Ethanol creates challenges for lawn mower fuel systems because it absorbs moisture from the air, can contribute to fuel degradation during storage, and may accelerate wear in older fuel-system components that were not designed for alcohol-blended gasoline. These issues show up more quickly in lawn mowers than in automobiles because mower fuel often sits for weeks or months in vented fuel tanks and carburetors.

Ethanol is hygroscopic, meaning it attracts moisture from the air. Over time, an ethanol-gasoline blend can absorb enough water that the ethanol and water separate from the gasoline and settle to the bottom of the tank—a process called phase separation (source: Bell Performance, "Protecting Your Clients From Ethanol Problems"). If that happens, the engine may draw a mixture of water and alcohol instead of gasoline, leading to hard starting, poor performance, or failure to start. Ethanol can also soften certain rubber, plastic, and fiberglass components found in older equipment or fuel systems that are not designed for prolonged exposure to alcohol-blended fuels.

There's also an energy difference. Ethanol contains less energy per gallon than gasoline, so E10 contains slightly less energy than ethanol-free gasoline, while E15 contains somewhat less still (source: Bell Performance, "Protecting Your Clients From Ethanol Problems"). For most homeowners the difference is modest, but it can result in slightly reduced run time per tank.

For these reasons, manufacturers such as Honda and Stihl recommend using gasoline containing no more than 10% ethanol (E10) in their gasoline-powered outdoor power equipment. A treatment such as Mix-I-Go Small Engine Formula helps stabilize ethanol-blended gasoline during storage and manage the moisture-related problems associated with ethanol.

Is E15 gas safe for small engines?

No. E15 — gasoline with 15% ethanol — is not approved for lawn mowers or any small engine, and the U.S. EPA prohibits its use in them (source: U.S. EPA, E15 misfueling regulation). The higher alcohol content increases every problem E10 already causes: more water absorption, faster phase separation, more solvency damage, and a hotter burn.

The trouble is that E15 is now sold at many of the same pumps as E10, sometimes labeled simply as "88 octane" or "Unleaded 88," which sounds like an upgrade. It isn't — for a mower it's a downgrade that can void your warranty. Check the pump label before you fill a gas can, and if a station only offers E15, get your small-engine fuel somewhere else.

Should you use ethanol-free gas in your mower?

If you can find it, ethanol-free gasoline is the safest choice for a lawn mower, especially one you store over the winter or use only a few times a season. With no ethanol in the fuel, there's no water absorption, no phase separation, and no alcohol attacking rubber and plastic parts. Many stations sell ethanol-free "rec fuel," and it's widely available in premixed cans for handheld equipment.

The downside is cost and availability — ethanol-free gas runs well above regular E10, and not every area has a station that carries it. For a lot of homeowners the practical answer isn't one or the other. Ethanol-free fuel removes the ethanol risk but not the aging problem: any gasoline, ethanol or not, starts to oxidize and lose combustion quality within a month or two of sitting. That's why the most reliable approach is fresh fuel plus a treatment like Mix-I-Go Small Engine Formula that both stabilizes the gas and neutralizes ethanol's effects — so whichever fuel you can get, it's protected.

How do you know if bad gas is hurting your mower?

The clearest signs of stale or ethanol-damaged fuel are hard starting, rough idling, stalling under load, and a gummy varnish smell around the carburetor. If your mower ran fine in the fall and won't start in the spring, old gas is the first thing to check — not the spark plug.

As gasoline ages and ethanol pulls in moisture, it leaves behind gum and varnish deposits that clog the tiny passages in a small-engine carburetor, and the phase-separated water can corrode metal and cause the engine to sputter or quit. Draining stale fuel and cleaning the carburetor is the common repair, and it's exactly the situation preventive treatment is designed to avoid. Fuel that's been stored more than about 30 days without a stabilizer should be treated as suspect.

What's the best way to protect your lawn mower's fuel?

The best way to protect a lawn mower is to start with fresh gasoline—either E10 or ethanol-free fuel—and treat it with a fuel stabilizer if the mower may sit unused for more than a few weeks. Stabilizing the fuel helps slow oxidation, keeps gasoline fresh longer, and helps reduce the moisture-related problems associated with ethanol-blended gasoline. This is especially important for seasonal equipment that spends long periods in storage.

Mix-I-Go Small Engine Formula is Bell Performance's treatment designed specifically for two- and four-cycle equipment, including push and riding mowers. It stabilizes gasoline during storage, helps manage the moisture associated with ethanol-blended fuel, helps protect fuel-system components, and helps keep carburetors cleaner by reducing gum and varnish deposits. Bell has been producing gasoline treatments under the Mix-I-Go name since 1927, and the Small Engine Formula is designed specifically for the storage and operating conditions common to lawn equipment.

For best results, add the recommended amount whenever you fill your gas can or mower, and treat the fuel before storing the mower for the off-season so stabilized fuel reaches the carburetor.

Mix-I-Go Small Engine Formula treat rates