Using the right type of gas and oil is necessary to keep your trimmer performing at its best.
STIHL String Trimmers use a 50:1 fuel mixture of gas and 2-cycle engine oil. An unleaded gasoline with a minimum 89-octane rating and maximum 10% ethanol content is mixed with a 2-cycle engine oil that is ISO-L-EGD and JASO M345 FD certified.
Take caution when working with gasoline and follow safety guidelines found in the operator’s manual. This includes working in a well-ventilated area and wearing eye protection.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
Gas and Oil to Use in STIHL String Trimmers
STIHL currently manufactures string trimmers with 2-cycle engines and 4-MIX engines. Each of these engine types requires a gas and oil fuel mix.
4-MIX engines combine the pros of a 4-stroke engine while using a gas and oil mix to keep its compact form. Unlike other 4-stroke engines on the market today, STIHL does not require a straight gas.
STIHL 4-MIX string trimmer models:
- FS 91, FS 91 R
- FS 111, FS 111 R, FS 111 RX
- FS 131, FS 131 R
Don’t make the mistake of adding straight gas to a STIHL 2-cycle or 4-MIX string trimmer or you will end up ruining the engine and most likely have to buy a new one.
Straight gas runs very dry and will cause the engine to seize and quit. Without oil added to the gas, the engine doesn’t get the lubrication it requires for internal parts to move freely.
Gas to Oil Fuel Mixture of 50:1
STIHL’s 2-cycle engine or 4-MIX engine on the string trimmers has one fill port for a gas and oil mix. The mix ratio is required to be 50 parts gasoline to 1 part oil (50:1 Mix).
| STIHL Gas to Oil Mix | 1 Gallon Gas | 2 Gallon Gas | 2.5 Gallon Gas |
|---|---|---|---|
| 50:1 | 2.6 oz Oil | 5.2 oz Oil | 6.4 oz Oil |
Type of Gas
For the best performance from your string trimmer, always use fresh unleaded gasoline with a minimum 89-octane rating and maximum ethanol content of 10%.
Because gas begins to break down as soon as 30 days after purchase, you should consume the fuel within 30 days and no longer than 60 days when adding a fuel stabilizer to the fuel.
Ethanol-based gasoline attracts moisture from the air to the fuel leaving behind a varnish in the fuel system. Gummy deposits will clog fuel components and prevent the carburetor from working sufficiently.
This can result in problems with your trimmer not starting, dying, or experiencing a loss of power.
Never use gasoline that has a greater ethanol content of 10% like those sold as E15, E30, and E85. These types of fuel contain up to 15%, 30%, and 85% of ethanol respectively. A low ethanol content or ethanol-free fuel is best.
Type of Oil to Use
STIHL string trimmers require a premium 2-cycle oil that is ISO-L-EGD and JASO M345 FD certified. STIHL offers its own branded oil: High Performance or HP Ultra for your string trimmer.
You can also use other brands as long as they are ISO-L-EGD and JASO M345 FD certified.
Two-cycle oil is different from automotive engine oil. Do not use standard engine oil in your string trimmers.
Pre-Mixed 2-Cycle Fuel
Another option to power your trimmer is with pre-mixed fuel. This is a good choice as it is an ethanol-free option.
It’s also convenient as it comes in a canister that can be stored on the shelf ready to go when you need it. No mixing is involved! Just shake and pour.
You can find pre-mixed fuel at your local STIHL dealership.
| Manufacture | Gas to Oil Mix | 2-Cycle Oil | Premixed Fuel |
|---|---|---|---|
| STIHL | 50:1 | High Performance HP Ultra | STIHL MotoMix |
How to Mix Gas
Determine how much fuel mix you want to prepare. Refer to the chart above for the amount of 2-cycle oil to add to the gas.
- Remove the cap from an approved gas can. Add unleaded fuel (minimum 89 octane rating & maximum 10% ethanol content) to a gas can.
- Add the appropriate amount of 2-cycle oil to the gas can.
- Replace the cap.
- Gently shake the fuel and oil until they are mixed.
- Add to the string trimmer’s fuel tank.
Always mark your gas can with the type of gas or gas/oil mix in the can. Mistakenly adding the wrong fuel to a piece of equipment can damage it.