Can you use 2-cycle oil in a lawn mower is a perfectly reasonable question to ask.
Maybe you have got a new 4-stroke mower and have some 2-cycle oil left over.
Or maybe you have put 2-cycle oil in your mower and realized your mistake too late.
You certainly aren’t the first person and won’t be the last person to have this question.
So let’s find out the answer…
Can You Use 2-Cycle Oil In A Lawn Mower?
You can, but you shouldn’t! Using 2-cycle oil in the oil fill of a normal 4-cycle lawn mower will damage the engine as it will burn out quickly and the engine will be running without oil. Using 2-cycle oil and fuel mix in the gas tank of 4-cycle lawn mower is less harmful initially but will also damage the engine if carried out over a long period of time.
Two-Cycle Oil and 4-Cycle Oil – What is the Difference?
Before diving into the title question, it is important to understand how 2-cycle oil is different from normal motor oil (4-cycle oil).
These two types of oil have very different formulations and do very different jobs.
At its most basic level, 2-cycle oil is much thinner and is made to be mixed with gas.
Two-cycle oil (also referred to as 2-stroke oil), is usually mixed with fuel at a ratio somewhere around 40/50 units of fuel to 1 unit of oil.
They are known as ‘once through’ oils.
Once in the engine, it will travel through the crankcase, lubricating the main bearings, rings, pistons and cylinder walls as it does so.
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It then enters the combustion chamber, where it burns with the fuel and is then passed out of the exhaust.
On the other hand 4-cycle oil is not meant to be mixed with fuel and stays in the crankcase and circulates continuously for thousands of miles/kilometres.
It coats parts that contact each other and contains detergents to keep any dirt in the oil rather than seeing it coat the inside of the engine.
A 4-cycle oil is usually much more viscous than a 2-cycle oil and is often a multigrade oil, rather than single grade as 2-cycle oil is.
Nearly all lawn mowers made in the last decade or so are 4-cycle mowers. Mowers made before this might be a 2-cycle.
Ok so let’s look at all the possible scenarios when it comes to using 2-cycle oil in a lawn mower.
Putting 2-Cycle Oil in the Crankcase of a Lawn Mower
If you put 2-cycle oil into the crankcase where regular motor oil goes, it might work initially but in no way will it be good for your lawn mower.
As we outlined above 2-cycle oil and 4-cycle oil have some very fundamental differences in their makeup.
And they are specifically designed for different types of engines.
Using 2-cycle oil in a 4-cycle lawn mower will ultimately damage the engine, because:
- 2-cycle oil is a ‘once through’ oil. It is designed to burn and be emitted through the exhaust. So it would burn out quickly and run the engine out of oil. The likely result would be an engine failure.
- 2-cycle oil doesn’t have the detergent contained in 4-cycle oil, which protects a 4-cycle engine.
- 2-cycle oil is much thinner and won’t lubricate the parts of the engine properly.
Two-cycle oil is not designed for the repeated use that a 4-stroke engine needs.
If you have accidentally added 2-cycle oil to your lawn mower, drain the crankcase and pour in the proper oil.
Two-cycle oil has different components from 4-cycle oil for a reason!
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Mixing 2-Cycle Oil With Fuel in the Gas Tank of Your Mower
Now let’s deal with the other possible scenario.
Maybe through force of habit, you have absent-mindedly poured some 2-cycle oil into the gas tank of your mower.
Or maybe you already have some 2-cycle fuel mix in your possession and want to know if you can pour it into the gas tank of your lawn mower.
This scenario is less harmful than using 2-cycle oil in the crankcase, but it still isn’t something I would recommend doing with any regularity.
Most 2-cycle engines use fuel-to-oil ratios of 40:1 or 50:1.
If you have 2-stroke oil and fuel mix made up like this, it is such a small percentage of oil that it is unlikely to cause any harm to a relatively small 4-cycle engine like that of a lawn mower if used on a rare occasion.
However if you have a lot more 2-cycle oil in the fuel than a 40:1 or 50:1 ratio, or if you are considering running your lawn mower regularly on a 2-cycle oil/fuel mix, it will still likely have some damaging long-term effects.
Namely the burning of the oil in the gas will create extra waste build-up in the combustion chamber, which could clog up the valves etc and will foul the spark plugs.
If you have a lot of leftover 2-cycle fuel mix and you are determined to use it, you should dilute it heavily with 4-cycle oil and gradually get rid of it that way.
But overall, it isn’t recommended.
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Putting 2-Cycle Fuel Mix in the Crankcase of a Lawn Mower
I wasn’t even going to include this scenario as I thought it was so unlikely, but then when I was researching this post I saw that someone had actually done it, so I will include it as a cautionary tale.
It probably doesn’t need saying but don’t ever put gas in the oil fill tube!
If you put 2-cycle oil/fuel mix in the crankcase and run the engine it will blow the engine.
Drain everything out completely and thoroughly and put 4-cycle oil in the crankcase!
Final Thoughts
The long and short of it is that 2-cycle oil has a very different composition to 4-cycle oil, as you can see from the infographic above.
That is because 2-cycle engines and 4-cycle engines are themselves very different.
Essentially 2-cycle oil burns off with the fuel, whilst 4-cycle oil stays in the crankcase and circulates continuously.
The key points are:
- 2-cycle oil should not be used in the crankcase of a (4-cycle) lawn mower. It will have a damaging effect on the engine.
- Using 2-cycle oil and fuel mix in the gas tank of a mower will do less harm initially, but prolonged use will also not be good for your mower long term, so avoid this too.
Two-cycle oil tends to be more expensive than 4-cycle oil, so that is another reason to avoid it!
Hope this clears things up.