Can I Use 10w40 Oil In My Lawn Mower? (Explained)

Oil can on bench

Even the most organized person can sometimes find themselves out of their regular lawn mower oil without realizing it.

That is why sometimes people ask the question, can I use 10w40 oil in my lawn mower?

Because a multigrade oil like this is often something they have to hand.

But before you get mowing you want to know if it will it cause any damage to your mower.

Well, let’s find out…

Can I Use 10w40 Oil In My Lawn Mower?

It is fine to use 10w40 oil as a temporary solution in your lawn mower if you are out of your normal oil. However prolonged use may cause excessive wear and tear on your engine and for long-term use it is always advisable to use a similar oil to that recommended by the manufacturer.

10w40 Oil Explained

So first of all let’s take a quick look at what exactly 10w40 oil is.

10w40 oil is a multi-viscosity oil. The number before the ‘W’ is the viscosity of the oil at a low temperature, ie before you start the engine of your lawn mower.

Essentially it tells you how the oil flows when it is cold.

The 10 means 10w40 oil remains very thin at low temperatures. In fact, the ‘W’ in the title stands for winter.

This is better when you have machinery that needs to be operated in cold temperatures as it reduces the warm-up time needed, meaning you can start whatever job it is you are wanting to do much quicker.

On the contrary, the 40 indicates how the oil flows when it is thick. A 10w40 oil will behave in the same way as a regular SAE 40 oil when it is hot.

In short that means when your engine is running at a normal operating temperature it will be quite thick. 

But where does that leave us when it comes to using it in a mower?

There are people who use 10w40 oil in their mowers and have done for years without a problem, and there are others who would never use it.

That makes it all the more difficult to decide who is right when considering whether to use it in your mower!

So let’s take a look at both sides of the argument.

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The Arguments Against Using 10w40 Oil in Your Mower

Oil can on bench

The main problem with 10w40 oil is how thick it is when the mower engine is running.

Its viscosity will prevent the oil from flowing and splashing through the smaller passages in the engine as well.

That means it won’t be able to help with heat dispersion and lubricate the engine as well as a 30-weight oil.

This means there will be more strain and wear on the engine. Which obviously, in the long run, isn’t good.

Furthermore nearly all domestic lawn mowers are air-cooled as opposed to liquid-cooled.

In the high heat of an air-cooled engine, a 10w40 oil is more likely to break down and simply become a mass of goo.

So really the volume and viscosity of 10w40 oil is the main concern.

In situations like this it is easy to say you should consult the manual of your mower to find out what oil can be used, but that really is what you should do!

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The Arguments For Using 10w40 Oil in Your Mower

10w40 oil suffers a bit from the bad reputation it gained in the past.

Back in the 1970s and 1980s, you would find warnings in manuals that 10w40 oil was not suitable for mower engines, mainly for the reasons outlined above.

Modern 10w40 is much different and of much higher quality, and should not cause issues with engines as it once did.

Nowadays lawn mower engines are so low-tuned and made with such low tolerances that I genuinely think oil quality isn’t as important as it once was.

It will not rev past around 2,500rpm, which is nothing to a small engine with light components.

Increasingly mower manufacturers are now recommending a 10w30 oil rather than a straight 30, something we explored when we answered the question Can I Use 10w30 Instead Of SAE 30 In My Lawn Mower?

Changing up (or down) one viscosity grade should not cause any real problems with the improved quality of modern oil.

The 10w40 oil will just remain a little more viscous when it is hot.

What is likely to cause more problems for your mower than using 10w40 oil is not keeping up a regular maintenance schedule.

If you use 10w40 oil, remember to keep an eye on the oil levels as it will burn off oil faster.

Also whilst the oil will be thicker at operating temperature it will still be slung around the machine, and whilst it might not lubricate quite as well the only thing you might notice is slight power loss when it is cold.

Finally, I previously I have mentioned that you should always consult the manual of your mower about what oil you need to use, but when you do this there is one thing you always need to keep in the back of your mind.

The manufacturers often recommend their own higher-priced, highly specialized oils to use in their machinery.

No doubt these oils will be very well suited to the machinery in question, but it is also a way of getting a bit more money from the consumer!

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Final Thoughts

Can I Use 10w40 oil in my lawn mower infographic

I am aware I have somewhat stood in the middle here, by simply giving you either side of the argument, as demonstrated by the infographic above.

My answer would be yes, you can use 10w40 if you don’t have your normal oil, however, I would not make a habit of it as it most probably will cause additional wear and tear on the engine with prolonged use.

In the long term it is always better to get the right oil, especially if you are using ethanol-based gas.

But if you need a short-term fix, you certainly can use 10w40 oil in your lawn mower, as using some oil is much better than using no oil!

Hope this helps!

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