When it comes to choosing the right fuel for your lawnmower, it is important to use clean, fresh, unleaded gasoline with a minimum of 87 octane/87 AKI (91 RON) . Gasoline with up to 10% ethanol (gasohol) or up to 15% MTBE (methyl tertiary butyl ether) is acceptable. However, it is important to avoid unapproved gasoline, such as E15 and E85, as well as fuels with more than 15% ethanol, as they can damage the engine components and void the engine warranty. If you are operating at high altitude, a minimum 85 octane / 85 AKI (89 RON) gasoline is acceptable. It is also important to note that you should not mix oil in gasoline or modify the engine to run on alternate fuels. For 2 Stroke Cycle Spark Ignited Engines, you should always mix a high-quality, 2-cycle oil, such as Briggs & Stratton lawn mower oil, at a 50:1 gas to oil ratio. Finally, to keep your fuel fresh, you can add fuel stabilizer according to instructions and run the engine for 2 minutes to circulate stabilizer through the fuel system before storing your lawn mower or equipment in a clean, dry place.