The problems we most frequently encounter with Stihl tools are NOT covered by the warranty. Incorrect fuel used. The fuel should not contain alchohol. Incorrect oil used. We use only Stihl oil in our equipment.
Customers sometimes use some other brand, or use the incorrect mixture.
Incorrect oil will cause all kinds of problems from plugged spark arrestor screens to poor operation to internal engine damage. Fouled spark plugs. Your owner's manual instructs you to use only fresh fuel. Further, it instructs you to completely drain fuel before storing. Fuel left in a machine over months will cause deposits not covered by warranty. 5.