I use 89 octane, as that is what Stihl specifies. Although I'm sure going higher is fine. I had a very long but simple conversation with a Stihl rep one night at a party. He said the NUMBER ONE reason for all small engine repairs is by far fuel issues. You must be strict and not mess with incorrect ratios. At the fuel pump, I fill each 2 gallon can to exactly 2.000 on the meter (or maybe 2.001 or .002!) Use MotoMix or a similar high-octane, ethanol-free mix for storage or a few times a year for maintenance. Good to go.