Every Stihl chainsaw uses fuel that mixes gas and oil together. These chainsaws don't operate with a separate oil reservoir and oil pump the way an automobile does. They need lubrication to come from the fuel source to keep the crankshaft and piston lubricated. Without the proper oil and gas ratio, the chainsaw will overheat and break down.
Stihl Chainsaw Oil
Stihl chainsaws operate regularly at incredibly high speeds — 12,000 RPMs and above, much higher than a car. This speed generates enormous heat and friction inside the cylinder and around the gaskets. If the engine can't cool off and protect the metal against this heat with oil, the gaskets will blow out and the piston will seize up. For this reason, use only high-quality engine oil in the fuel mix. Stihl recommends using its own brand of engine oil. However, this isn't an absolute requirement; any air-cooled, high-quality two-stroke engine oil will keep the engine protected.
Stihl Chainsaw Gas
When the chainsaw ignites the fuel, the leftovers vent out the muffler. This also creates a lot of heat and pressure in the engine. To solve this problem, use a high-quality, high-grade gasoline with the mix. This will prevent the gas from burning improperly inside the cylinder and will ensure that all the gas will get burned cleanly. Stihl recommends using a fuel with a grade above 89 octane. Never use fuel that has an additive or alcohol base to the fuel, such as ethanol or gasohol. These fuels will damage the plastic in the carburetor.
All Stihl chainsaws have a set gas to oil ratio when using high-quality two-stroke engine oil. This is set at a ratio of 50 parts gas to 1 part oil. However, real-world situations often occur that may force you to change the mixture ratio slightly. If you use other lower-quality engine oils not recommended by Stihl, drop the ratio to 30:1. Increase the mixture slowly up to 40:1 if the engine isn't burning or running smoothly. Continuously monitor the engine performance during operation and make adjustments to the ratio as needed, but never go above 50:1.
Mixing the Gas
The oil must break apart inside the gas for it to reach all of the moving parts on the Stihl engine. If the gas is mixed improperly, the oil won't be blended thoroughly with the gas, and engine damage may occur. Always pour in the oil first, making sure the fuel container is cleaned out from the last mix. Pour the gas in on top of the oil, close the container and shake it up for at least one minute. Shake the gas and oil together before each refueling stop.