40:1 vs 50:1: Can You Run 50:1 Instead of 40:1?

40:1 vs 50:1 comes down to 25 percent more oil. A 40:1 mix is 3.2 oz of 2-stroke oil per US gallon of gas (25 ml per litre); a 50:1 mix is 2.56 oz per gallon (20 ml per litre). Running 40:1 in a machine built for 50:1 is safe and only costs you some smoke. Running 50:1 in a machine built for 40:1 is safe only when the engine maker says so, usually because a better grade of oil replaced the one the old spec was written around. Everything else on this page is the detail behind those two sentences: the numbers side by side, which machines sit at each ratio, how to convert a can you already mixed, and what to do when one can has to feed both.

The numbers side by side

	40:1	50:1
Oil per US gallon	3.2 oz	2.56 oz
Oil per litre	25 ml	20 ml
Oil for 2.5 gallons	8 oz	6.4 oz
Oil for 5 gallons	16 oz	12.8 oz
Oil for 5 litres	125 ml	100 ml
Oil as a share of the gas	2.5%	2%
Gas one 2.6 oz bottle treats	0.81 gal	1.02 gal

Two ways of reading the same gap: 40:1 has 25 percent more oil than 50:1, and 50:1 has 20 percent less oil than 40:1. The second framing is the one that matters for engine safety, because it describes what a 40:1 engine loses when someone fills it from a 50:1 can.

The “percent” figures are what European manuals and oil bottles print. 2% oil is 50:1 and 2.5% is 40:1, measured as oil against the volume of gas. The mix chart has every other ratio in the same format.

Which machines run which

50:1 is the current standard for air-cooled handheld equipment. Stihl specifies 50:1 for every gasoline tool it sells, 4-MIX engines included. Husqvarna specifies 50:1 (it writes it as 2%) for its saws, trimmers and blowers up to 75 cc. Echo specifies 50:1 across its current range. Most carbureted two-stroke outboards are 50:1 too, with a TC-W3 marine oil.

40:1 is the older-equipment and budget ratio. Chainsaws and trimmers built from the 1970s to the early 1990s by Poulan, Homelite, Craftsman and McCulloch commonly carry 40:1 decals, as do plenty of budget handhelds and some augers and hedge trimmers sold today. On dirt bikes, KTM’s manuals give a 40:1 to 60:1 range and say to use 40:1 when in doubt, while the Japanese bikes sit nearer 32:1 (the dirt bike premix guide has the brand table).

For a count, the 2-stroke mix ratio statistics tally the ratio printed by the manufacturer for 61 current saws, trimmers, blowers and dirt bikes: 59 state 50:1, and the two exceptions are Yamaha motocross bikes at 30:1.

Running 40:1 in a 50:1 engine

This is the safe direction. The engine gets more oil than it needs, and the cost shows up as:

• More blue smoke at start-up and idle.
• A spark-arrestor screen and exhaust port that load up with carbon faster. On a saw or trimmer that is used hard, plan on cleaning the screen more often.
• A spark plug that fouls sooner, especially on machines that idle a lot.
• A very slightly leaner fuel charge. The extra oil displaces gasoline: at 40:1 the mix is about 97.6% gas, at 50:1 about 98.0%. Half a percent is inside normal carburettor tolerance, and no handheld needs re-tuning for it.

None of those damages an engine. That is why 40:1 is the right answer when you own a 40:1 saw and a 50:1 trimmer and want to keep one can: mix the whole can at 40:1 and both machines are protected.

Running 50:1 in a 40:1 engine

This is the direction that breaks engines, but not always. Two cases:

The maker has approved it. Ratios moved over the decades because oils improved, not because engines stopped needing protection. JASO FC and FD oils, and synthetics in particular, hold a film at a lower treat rate than the mineral oils older specs assumed. Echo is the clearest example: older manuals printed richer ratios such as 32:1, and Echo’s current manuals say Echo-branded 2-stroke oil may be mixed at 50:1 for all Echo engines sold in the past, regardless of the ratio in those manuals. If your maker has said something similar, follow it, with the oil it names.

The maker has not. Then 50:1 is a 20 percent cut in lubrication on an engine whose tolerances, port timing and cooling were designed around 40:1. Too little oil does not announce itself. It shows up as a gradual loss of power over a few tanks, then a scored piston skirt you can see through the exhaust port, then a seizure. On an old saw the repair often costs more than the saw.

The bottle can mislead you here. A premium oil printed “50:1” or even “100:1” is telling you what that oil can do, not what your engine was approved for. When the bottle and the engine disagree, the richer of the two is the safe choice. The AMSOIL Saber guide covers the same argument at 100:1.

Converting a can you already mixed

People rarely ask this question before mixing. They ask it holding a full can of the wrong ratio. The arithmetic is simple because the ratio is measured against the gas:

• 50:1 to 40:1: add oil. Each gallon of gas in the can needs 0.64 oz more oil. That is 1.6 oz for a 2.5 gallon can and 3.2 oz for 5 gallons, or 5 ml per litre in metric. Add it and shake the sealed can.
• 40:1 to 50:1: add gas. The oil already in the can will treat 25 percent more gas at 50:1, so add a quarter of the can’s gas volume: 1 quart per gallon, or 250 ml per litre. A full 2-gallon can of 40:1 needs 2 more quarts of gas, so it has to go into a bigger can first.

If the can is more than a month old, mix fresh instead. Converting stale fuel only gives you stale fuel at a better ratio.

Measuring either ratio

The common error at both ratios is not choosing the wrong number but pouring the right number badly. Most single-use oil bottles are 2.6 oz, sized for one US gallon at 50:1. One of those in a gallon at 40:1 leaves you lean: it only treats 0.81 gallon at that ratio. A ratio bottle with both scales printed on it, or a syringe for small batches, removes the guesswork; the measuring tools guide compares them. For any amount that is not on the chart, the calculator below is preset to 40:1 and switches to 50:1 in one tap:

Fuel → oil / Oil → fuel

Fuel → oil Oil → fuel

Amount of fuel

Gallons (US) Litres

Equipment (optional) Choose equipment… Chainsaw (50:1) String trimmer / weed eater (50:1) Leaf blower (50:1) 2-stroke lawn mower (50:1) Dirt bike / motorcycle (40:1) Outboard motor (50:1)

Mix ratio

100:1 50:1 40:1 32:1 25:1 20:1

Custom ratio (X : 1)

Calculate Reset Copy link to this mix

Add this much oil

16 oz

≈ 473.18 ml

Add 16 oz of 2-stroke oil to 5 gal of gas at a 40:1 ratio.

How to decide in thirty seconds

1. Read the fuel cap or tank decal. If it gives a ratio, use it. Stop here.
2. No decal? Find the manual. Most makers publish PDF manuals by model number. The model plate is usually under the rear handle or on the engine shroud.
3. No paperwork either? Mix 40:1. Guessing rich costs you smoke; guessing lean costs you a piston.
4. One can for several machines? Mix at the richest ratio any of them needs.
5. Dirt bike or outboard? Treat the ratio as part of the jetting or the oil system. Pick the manual’s figure, stay on it, and read the 50:1 page or the 40:1 page for the batch tables at each.

The engine never cares which ratio sounds more modern. It cares whether the oil film on the cylinder wall is thick enough for the heat it makes, and the person who wrote the decal knew that number better than any forum thread.