Stihl Trimmer Fuel 40:1 vs. 50:1 (Field Comparison)
For a STIHL two-stroke trimmer, use the gasoline-to-oil ratio stated in that exact model’s manual. Most require 50:1; use 40:1 only when the manual specifies it. At the same gasoline volume, 40:1 adds 25% more oil, but that does not reliably protect an engine with a fuel, air, or carburetor problem.
More oil can sound like cheap insurance when a trimmer is old, hard to start, or losing power. But the ratio is a specification, not a general durability setting. A richer mix can increase smoke and deposits, while too little oil can damage an engine. Neither choice fixes stale fuel, a clogged filter, or an air leak.
I start with the model number and its operator manual, then measure the mix rather than estimating by eye. This matters when several tools share a shed: two similar-looking trimmers may have different requirements, and a ratio remembered from another engine may not apply. The comparison below shows what the numbers mean and how to use them safely.
Equipment Setup & Safety Prep
The fuel ratio is the volume of gasoline compared with the volume of two-stroke oil. A 50:1 mix uses one part oil for every 50 parts gasoline; 40:1 uses more oil for the same gasoline volume. The correct choice is the one listed for your trimmer, not whichever ratio seems more protective.
How much oil goes in each mix?
| Gasoline | 50:1 mix | 40:1 mix |
|---|---|---|
| 1 L | 20 mL oil | 25 mL oil |
| 1 US gallon | 2.6 US fl oz oil | 3.2 US fl oz oil |
| 5 L | 100 mL oil | 125 mL oil |
These amounts are for the stated gasoline volume. Use a graduated container made for mixing fuel, not a kitchen measure or an unmarked bottle. Add the amounts your manual specifies, mix thoroughly, and label the container with the ratio and date.
At equal gasoline volume, the 40:1 mix contains 25% more oil than 50:1. That difference does not mean 40:1 is a universal upgrade. If your manual calls for 50:1, adding more oil can contribute to smoke, spark-plug fouling, and carbon deposits.
Check fuel and oil compatibility
Follow the manual for gasoline octane and ethanol limits. STIHL generally specifies gasoline with at least 89 AKI and no more than 10% ethanol, but check the manual for your model and market. AKI is the octane rating shown at many North American pumps.
Use quality air-cooled two-stroke oil that meets the manual’s requirements. STIHL recommends its own two-stroke oil. Do not use four-stroke engine oil or bar-and-chain oil in the fuel. If you are unsure whether a fuel or oil is suitable, check its label and the trimmer manual before mixing.
Prepare the trimmer and work area
Before starting, check the guard, handle, harness, throttle trigger and interlock, stop switch, cutting attachment, fasteners, and fuel system. Look for cracks, loose parts, fuel leaks, or damaged lines. Wear eye and face protection, hearing protection, gloves, long pants, and sturdy footwear suited to the job.
Start on stable ground with the cutting attachment clear. Keep people outside the exclusion distance stated in the manual, and direct debris away from people, windows, and vehicles. Trimmer cutting attachments can throw objects, so avoid striking rocks, fences, and other hard surfaces.
Next step: Confirm the model’s ratio and fuel rules before pouring fuel. If the manual is missing, find the model identification and consult STIHL’s official manual or an authorized dealer.
Field Cutting/Landscaping Operation
A field comparison is useful only when the machine is set up correctly. Use the prescribed fuel mix through the work session, follow the model’s starting instructions, and watch for changes in sound, power, or exhaust. A different ratio will not correct a clogged fuel path, a lean carburetor setting, or an air leak.
Starting, cutting, and refueling
Follow the cold-start and warm-start steps in the manual. Controls and procedures vary by model, so do not rely on a generic choke position or starting sequence. If the engine will not start after the specified steps, stop and check the manual’s troubleshooting section rather than repeatedly pulling the starter.
During trimming, maintain control of the tool and cutting attachment. Stop if you notice abnormal overheating, persistent misfiring, loss of power, unusual exhaust, or fuel leakage. Let the engine cool before refueling, and refuel away from flames, sparks, and other ignition sources.
Field scenarios: ratio versus fault
These examples are diagnostic situations, not tests proving that one ratio lasts longer than the other.
- A trimmer runs poorly after sitting: Old or contaminated fuel is a likely place to begin checking. Mixing fresh fuel at 40:1 instead of the specified 50:1 does not refresh stale gasoline or clear a blocked carburetor passage.
- A trimmer loses power under load: Check the air filter, cooling area, cutting attachment, and fuel supply as directed in the manual. If the problem continues, an air leak or carburetor issue may need service. Extra oil is not a reliable fix.
- Two trimmers need fuel: Read each manual or label before mixing. Do not assume both take the same ratio because they share a brand, look similar, or use the same type of cutting head.
These checks help separate a mix-up from an engine fault. Keep a known, correctly labeled fuel batch for each required ratio, and do not combine fuel of uncertain age or composition.
Maintenance & Sharpening/Tuning
Routine cleaning and inspection help the trimmer run as designed, but they do not replace the specified fuel mix. A lean-running engine has too much air or too little fuel for its operating needs, and may overheat. Causes can include fuel starvation, an air leak, stale fuel, or incorrect carburetor adjustment.
What extra oil can and cannot do
Two-stroke oil lubricates engine parts through the fuel mixture. Using less than the specified amount can raise the risk of inadequate lubrication and engine damage. Using more than specified is not a dependable way to prevent seizure: it does not restore fuel flow, seal an air leak, or correct carburetor tuning. It can also increase smoke and deposits or foul the spark plug.
If the trimmer shows signs of overheating or repeated power loss, stop using it and let it cool. Check for obvious issues such as a dirty air filter or blocked cooling fins, following the manual. If the problem remains, have the fuel system and carburetor checked by a qualified service provider. Do not adjust the carburetor by a guessed number of turns or a generic engine-speed target.
Keep model-specific adjustments model-specific
Idle and maximum engine speeds vary by trimmer. Use the figures and adjustment method in the exact manual or authorized service data. Chainsaw specifications such as bar pitch, chain gauge, and bar-oil flow do not apply to a trimmer.
Keep the air filter, cooling fins, and spark-arresting screen clean and serviceable if your model has one. Also inspect the cutting attachment and fasteners. A dirty filter or cooling area can affect operation, but cleaning it does not make an incorrect fuel ratio acceptable.
Next step: If symptoms persist after basic manual-approved checks, stop guessing and arrange a model-specific inspection. A technician can check for fuel restriction, leaks, and correct carburetor settings.
Storage & Seasonal Care
Fuel ages, and storage guidance depends on the fuel, oil, and model. For reliable seasonal starts, use fresh fuel mixed at the specified ratio and follow the trimmer manual and fuel maker’s instructions for storage. Do not assume that adding oil makes old fuel suitable for another season.
Store fuel and trimmer safely
Keep mixed fuel in an approved, clearly labeled container away from heat and ignition sources. Label the ratio and date so you do not mistake 40:1 fuel for 50:1, or the reverse. Follow the container and product instructions for handling and storage.
For extended storage, follow the STIHL manual’s steps for draining or treating fuel and running the engine, if instructed. Procedures can differ by model. Before storing the trimmer, let it cool, check for leaks or damage, and secure it where children cannot reach it.
Quick ratio decision
| If your manual says… | Mix to… | Do not… |
|---|---|---|
| 50:1 | 20 mL per 1 L gasoline, or 2.6 US fl oz per US gallon | Switch to 40:1 as extra protection |
| 40:1 | 25 mL per 1 L gasoline, or 3.2 US fl oz per US gallon | Assume another STIHL model uses the same ratio |
| You cannot confirm the ratio | Pause and find the exact manual or contact an authorized dealer | Guess or use fuel mixed for a different tool |
Bottom line: The manual decides the ratio. Measure it accurately, use compatible fuel and oil, and diagnose poor running rather than trying to mask it with extra oil.
FAQ
These answers cover the key difference between the two mixes and the practical steps that help prevent fuel-related trouble. When a general answer conflicts with your exact model’s manual, follow the manual and seek STIHL service advice if anything is unclear.
Is 50:1 the right mix for every STIHL trimmer?
No. Many STIHL two-stroke trimmers call for 50:1, but check the manual for your exact model. Use 40:1 only if that manual specifies it.
Does 40:1 provide more lubrication than 50:1?
It contains more oil for the same gasoline volume: 25% more. That does not make it a suitable upgrade when the trimmer is designed for 50:1.
How much oil goes into one gallon at 50:1?
Use 2.6 US fluid ounces of oil per US gallon of gasoline. Measure with a graduated mixing container and check your manual.
How much oil goes into one gallon at 40:1?
Use 3.2 US fluid ounces of oil per US gallon of gasoline, if your model’s manual calls for 40:1.
Can I use 40:1 in a trimmer that specifies 50:1?
Do not substitute it as a precaution. Follow the specified 50:1 ratio; extra oil can increase smoke and deposits and does not fix a running fault.
Can I use 50:1 in a model that specifies 40:1?
Do not use a mix with less oil than the manual requires. Make the specified 40:1 mix or ask an authorized STIHL dealer for guidance.
Will adding more oil fix a trimmer that runs lean?
No. A lean condition can result from fuel starvation, an air leak, or incorrect carburetor adjustment. Stop using a trimmer that overheats or loses power repeatedly and have it checked.
What gasoline should I use?
Follow the manual’s octane and ethanol limits. STIHL generally specifies at least 89 AKI and no more than 10% ethanol; confirm the requirements for your model.
Can I use fuel left over from last season?
Do not assume it is still suitable. Follow the fuel and oil makers’ storage guidance and the trimmer manual. When fuel age or condition is uncertain, use fresh, correctly mixed fuel.
Only if both manuals require the same ratio and the fuel meets both models’ requirements. Label the container clearly; if the ratios differ, keep separate, marked mixes.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
