Gasoline is something you may not give a lot of thought to, but the type of fuel you use in your chainsaw makes a difference in the lifespan and in how well your saw runs.
I strongly advise using fresh gasoline with an octane rating of 89 or higher (mid grade or premium).
Most of the regular unleaded 87 octane gasoline that is available on the market today contains minimal amounts of detergent additives, which in four stroke applications keep pistons, valve stems, and fuel injectors clean. These same additives are needed in a two stroke engine to keep varnish, gum, and lacquer deposits from forming inside the engine.
Pressure from the automotive industry has caused the gasoline suppliers to blend the 89 to 91 octane gasoline range with greater amounts of detergents and anti-oxidizing additives to prevent formation of deposits and keep engines clean.
Lack of detergent additives in two stroke fuel will cause heavy buildup of varnish, gum and lacquer deposits on the piston surface and inside of the ring grooves. Heat transfer from the piston to cylinder will suffer. Once the ring starts sticking in the ring grooves, combustion flame and exhaust gases will leak past them, further overheating pistons causing galling or seizures.
Fuel older than 30 days will begin to form varnish and gum. When the varnish and gum are heated by the combustion, they combine and form the byproduct tar. The tar byproduct is the number one cause of blocked spark arrestor screens and stuck piston rings.
Since old gasoline won't vaporize well in the crankcase, the fuel will puddle in the crankcase, causing spark plug fouling, engine flooding and hard starting. Old, stale, oxidized fuel will also quickly form gum, varnish and deposits inside the carburetor and fuel system components. This is the most commonly found issue at many dealerships at the beginning of the season, primarily with products that are used seasonally.
"Moral of the Story"
Use only fresh mid-grade or premium gasoline in your chainsaw engines and other 2 cycle engines. It'll make your chainsaw last longer and run better with less maintenance and repair hassle.