Only purchase the amount of fuel that will be used within 30 days
Fuel degrades over time. Deterioration begins with the evaporation of the most volatile compounds. Once evaporation reaches a certain point, it will be difficult/impossible to start the machine. As more compounds evaporate, the fuel forms brown gummy deposits in the system. Given enough time, the gummy deposits will become a hard varnish. Gummy deposits and varnish can clog passages in the carburetor, preventing the engine from running or causing poor engine performance (surging, loss of power, stalling, etc.). Deposits can also cause fuel to leak into the carburetor if they prevent the float needle from sealing.
Purchase unleaded gasoline with an octane rating of 87 or higher
Unleaded gasoline with an octane rating of at least 87 (test method (R+M)/2) is the recommended fuel quality for all Toro gasoline engines. Gasoline containing up to 10% ethanol (gasohol) or 15% MTBE (methyl tertiary butyl ether) by volume is acceptable. Ethanol and MTBE are not the same. Gasoline containing 15% ethanol (E15) by volume is not authorized. Never use gasoline containing more than 10% ethanol by volume, such as E15 (contains 15% ethanol), E20 (contains 20% ethanol), or E85 (contains up to 85% ethanol). Using unapproved gasoline can cause performance problems and/or engine damage that may not be covered by the warranty. Keep in mind that ethanol fuel blends absorb water from the atmosphere and can cause corrosion of fuel system components. Since most carburetors and fuel tanks are vented to the atmosphere in one way or another, nothing prevents the fuel from absorbing moisture over time. Using fresh fuel (less than 30 days old) will help prevent water absorption from becoming a problem.
Consider using ethanol-free gasoline (E0)
Ethanol-free gasoline will significantly reduce the amount of moisture that the gasoline can absorb from the atmosphere. Many regions of the country have ethanol-free gas available, and it is easy to find. Search the Internet for "ethanol-free gasoline."