The ratio of fuel stabilizer to gasoline varies depending on the product and manufacturer. Consult the manufacturer's instructions for exact details. Sta-bil, one of the major product manufacturers, produces products with ratios ranging from 1 oz. per 6 gallons to 1 oz. per 10 gallons. An ounce of Amsoil Gasoline Stabilizer treats 2 1/2 gallons of gasoline. Whichever product you use, following proper instructions is vital.
Treat the Gas in the Mower
Fill the lawnmower gasoline tank nearly full with gasoline after the last use in the fall. Add the proper amount of stabilizer and close the gasoline cap. The full tank of fuel prevents rust and corrosion on the inside of the gas tank.
Treat Stored Gas
If you have a can of gas you'll be storing until the next spring's use, it should also be treated. Treat any gas that is stored more than three months. This is not necessary if you will use the gasoline in a snow blower or other small engine in the next few weeks.
Winterize the Mower
Change the oil after the final lawn mowing of the fall. Draining the oil while the engine is warm clears the oil and any accumulated sludge from the engine crankcase. Fill the engine with 30-weight oil according to the mower manufacturer's instructions. Dispose of the old oil properly. Clean or dispose of the old air filter.
Clean It Up
Clean the external parts of the motor and the underside of the mower deck before fall storage. This is also an opportunity to sharpen the blade and replace the spark plug. All of these steps prepare the lawnmower engine for easy starting next spring. For safety, leave the spark plug wire disconnected until spring. Remember to reconnect the spark plug wire before attempting to start the engine in the spring.