Stihl manufactures string trimmers for both home and professional use, and although some are corded or battery-powered, the majority are powered by gasoline. Like other manufacturers, Stihl offers a variety of models with two-stroke engines, but some Stihl trimmers have a specialized four-stroke engine that Stihl calls "4-Mix." It takes a mixture of gas and oil, just like conventional two-stroke engines do. The mixing ratio for most Stihl trimmers is 50-to-1.
Lubricating Two-Cycle Engines
Although less efficient than than their four-cycle counterparts, two-cycle engines generate more power and are the norm for small gas-powered tools, such as string trimmers. All internal combustion engines need the lubrication of engine oil to counteract the friction generated by metal parts as they rub against each other at high speeds and temperatures. You can't add oil to the crankcase as you do with a four-cycle engine, or the oil would flood the piston chamber and blow out of the exhaust port during combustion. Instead, the oil is added to the fuel and burns along with it during normal engine operation.
Stihl Gas Trimmer Engines
Only a few manufacturers make string trimmers with conventional four-cycle engines, and Stihl isn't one of them, although it does offer models with the 4-Mix engine.This engine combines the features of four-cycle engines, which include better efficiency and fewer atmospheric pollutants, with the convenience of gas/oil mixtures typical of two-cycle engines. The 4-Mix engines have a bypass channel in the cylinder head that allows for complete movement of the oil-rich fuel throughout the cylinder, which ensures complete lubrication. This technology eliminates the need for an oil tank and oil pump, which both make the tool bulkier and heavier.
The Standard Fuel/Oil Mixture
All of Stihl's recent trimmer models, including those with 4-Mix engines, rely on a 50-to-1 fuel oil ratio. That ratio translates to 2.6 fluid ounces of oil for every gallon of gasoline; if you're mixing 5 gallons of gas, you would add 12.8 ounces of oil. You seldom have to measure out the oil -- most two-cycle oil comes in calibrated containers, and you can determine the amount you're using by reading the scale on the side. To mix fuel for your Stihl trimmer, first pour the fuel into an approved container, then add the gas and shake. Stihl recommends mixing only what you can use in 30 days.
Things to Keep in Mind
It's always a good idea to double-check the fuel oil ratio for your model by consulting the manual, which is available on the Stihl website. Some older models, such as the FS 81, 86 and 106, take a 40-to-1 mixture, and using the leaner oil mixture for newer machines could damage your trimmer -- although it would take time. Use only two-cycle motor oil, which is not as heavy as four-cycle oil and burns more efficiently. Before removing the gas cap to fuel your trimmer, clean grass and dirt from around the opening to prevent it from falling into the gas tank and contaminating the fuel.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.