10W30 vs 10W40

10W30 vs 10W40 – What Happens If I Use The Wrong Oil?

Welcome back to another article where we compare two different motor oils. In this one, we will be looking at 10W30 vs 10W40.

These two motor oils are very similar to each other but shouldn’t be used interchangeably.

We will go through the following sections while considering 10W30 vs 10W40.

Feel free to click on the relevant link above to jump to whichever section you want.

Let’s get started.

What Does Motor Oil Do?

If you have a car, you will at least be aware of the need to keep the engine topped up with motor oil. It’s one of the fundamental parts of learning to drive and basic maintenance.

However, you might not be quite so sure about what this liquid does.

Sure, there’s the oil cap on the top of your engine, but why is it important to keep the level topped up? Why does it need changing so often? Why can it be quite expensive? And – most importantly – what does it actually do?

What Motor Oil Does

The main function of motor oil is to keep the system lubricated.

Most oils used in machinery and manufacturing processes are used for this kind of thing. The oils used in an engine are no exception.

The oil is designed to form a layer between separate metal surfaces. This stops them from scraping against each other or otherwise getting in each others’ ways. If this was to happen, it would cause friction.

Engine oil functions as a lubricant between metal surfaces.

Friction can (usually) be thought of as the Most Wanted Criminal in terms of the engineering industry.

(Note: this isn’t always the case. For example, if tires didn’t create any friction, they would be effectively useless).

An engine should be as efficient as possible. To most of us, we can just imagine it as running smoothly. The smoother, the better.

In technical terms, this comes down to the amount of energy put into the engine compared to the amount of energy it produces, expressed as a percentage.


Nothing is 100% efficient, but the higher the efficiency, the better.

Why Does Motor Oil Need To Be Kept Topped Up?

Motor oil needs to be kept topped up so that there is always enough in the system.

In internal combustion vehicles (gasoline, diesel, etc), cars are fitted with oil systems. These include a sump, an oil pump, an oil filter, and plenty of passageways cut through the engine block. These passageways direct the oil flow through the engine block and keep everything lubricated.

If the level of motor oil was to get too low, there would no longer be enough oil to keep the engine lubricated.

It’s also worth pointing out that, in the same vein, too much motor oil can lead to issues of its own.

You should always aim to keep the oil level between the maximum and minimum marks on your dipstick.

It doesn’t have to be topped up to the “maximum”. As long as it’s between the two levels, you’re all good.

Make sure you check the oil level on flat ground, otherwise the sump will not be level. This will lead to an inaccurate reading when you put the dipstick in.

Why Does Motor Oil Need Changing?

Unfortunately, as the old saying goes, “nothing lasts forever”.

With use and general wear and age, motor oils degrade. Multigrade oils (such as the ones we are considering in this article, 10W30 and 10W40) usually degrade slightly faster than monograde oils (such as SAE 20).

This is because multigrade oils have additives in them, which can get chopped up by the force of the engine.

10w30 vs 10w40 motor oil change

Why Can Motor Oil Be Quite Expensive?

Some oils are more expensive than others. Usually, as price increases, so too does the quality of the oil.

In more expensive grades, you should expect to find a whole host of extra additives that will serve your engine in many more ways than just lubricating it. These can include cleaning agents, anti-rust agents, long-lasting viscosity modifiers, and many more.

It’s always worth checking what’s in the oil you’re buying. If the oil seems too cheap to be true, it probably is. However, likewise, it’s not always necessary to spend a fortune.

We would recommend purchasing mid to high-range motor oil. This is more likely to look after your car, meaning it will run better and for longer.

You’ll find that, in the long run, this will often be better for you financially than just throwing any old cheap oil in your engine.

How Do 10W30 vs 10W40 Compare?

Before we get into the “why”, we will briefly explain the difference between the two oils.

10W40 is “thicker” than 10W30 when the car is at running temperature. Some people might also refer to it as “heavier”. Neither of these terms is technically accurate, but they do help to visualize it.

In reality, we would say that 10W40 is more viscous than 10W30. By contrast, when thinking about 10W30 vs 10W40, we would say it is less viscous.

Viscosity is a term expressing something very similar to the “thickness” or “heaviness” of liquid, but with a little more scientific depth.

We will look into viscosity in the next section.

Another first thing to note when we compare 10W30 vs 10W40 is that very few cars will be designed to run on both oils. You should never, ever, ever go against what the owner’s manual says to put in the car, unless in exceptional circumstances where you are sure you know what you’re doing.

Manufacturers design engines to work as efficiently as possible with certain motor oil, or sometimes a range of motor oils. Going outside of this area can only lead to bad things.

What Is Viscosity?

Lots of people think of viscosity as either the heaviness or the thickness of a liquid. Although it’s broadly expressing the same thing, there is slightly more to viscosity than this.

This video from 2 Minute Classroom is a great introduction to viscosity.

As well as looking at the different flow behaviors exhibited by different fluids, this video also briefly touches on how viscosity changes (reduces) with temperature. For example, as honey warms up it becomes “runnier”.

This is particularly relevant to a car’s engine, so keep it in mind.

Click here to jump to physics.info for a full-on, in-depth explanation of what is going on.

Quoting from the above-mentioned site, we see that…

viscosity is the quantity that describes a fluid’s resistance to flow…

In terms of physics, we would say that viscosity expresses the ratio between the shear stress and the velocity gradient.

It might help to think of it in terms of internal friction. The more internal friction a fluid has, the more it tries to stop itself from flowing. This would mean it is a more viscous fluid.

If that’s all far too much information at one time, don’t worry – just think of it in terms of thickness. The “thicker” a liquid is, the more viscous it is.

Comparing honey vs water is a traditional comparison to explain this concept. If you had a glass of honey and a glass of water and you tilted them at the same rate, at the same time, you instinctively know that the water is going to flow much more quickly than the honey.

This is because honey is more viscous than water. Or “thicker”.

For a more in-depth video about viscosity and, specifically, how it changes with temperature, check out this one from Tutorials Point (India) Ltd.

10W30 vs 10W40 – What Do The Numbers Mean?

Now that we have defined viscosity, we come to think about how that impacts our subject for this article.

Both 10W30 and 10W40 are multigrade oils. This means that they have more than one grade. In this case (and almost every other), there are two grades. For example, in 10W30, we find an oil that behaves like SAE 10 in cold temperatures and like SAE 30 in normal operating temperatures.

The “grade” refers to the level of viscosity.

It’s important to know, here, that the grade is not a direct measurement of viscosity. They are purely relative. Grades are assigned by the SAE designated through four tests, which we will explain a little more about shortly.

engine oil 10w20 vs sae 10 sae 20

Take a look at this graph. Although it’s not a direct representation of how liquids behave, it’s useful for illustrative purposes.

Remember how we showed that viscosity in liquids decreases with temperature? You can see this on the graph.

The green and pink lines represent standard monograde oils, those being SAE 10 and SAE 20, respectively. You can see how these decrease in viscosity with temperature.

Using a wide variety of additives called viscosity modifiers (which are polymeric molecules), engineers can modify the characteristics of liquids such as motor oils. From that, we get a “multigrade” oil, which behaves like one grade at 0 degrees C, and like another at 100 degrees C.

With the graph, as an illustration, you can see that 10W20 behaves like SAE 10 at 0 degrees Celcius and like SAE 20 at 100 degrees C.

Why This Is Relevant

On a cold morning, when starting your car, the oil needs to get around the engine as quickly as possible. The thicker the oil is (the more viscous it is), the more power the oil pump needs to put out to shift it.

This means there is a greater load on your battery, which is something we don’t want.

Therefore, the oil should be as “thin” (have a lower viscosity) as possible when the car is starting up.

It also follows that, once the car is running and up to temperature, the oil should be relatively “thick” (viscous). This is because the moving metal parts inside the engine need as much protection as possible. The motor oil is what provides this.

More viscous oils work better once the engine is up to temperature, but are less efficient when the engine is warming up. For less viscous oils, the opposite applies.

This is why we have multigrade oils.

They can be as “thin” as possible when the engine is starting and warming up, before becoming as vicious as possible once it’s up to temperature.

10W30 vs 10W40 – What Does the “W” Stand For?

As we’ve just seen, there are two different parts to a multigrade oil’s definition.

The first grade refers to the temperature at 0 degrees C, representing “winter” (hint, hint, nudge, nudge) conditions. The second number is the grade at operating temperatures of 100 degrees.

The “W” stands for “winter”.

It just means that this is the oil grade when it’s cold.

Engineers aren’t the most inventive lot when it comes to names and definitions, sometimes.

You’ll usually see oils written as 10W-30 or 10W-40, the difference being the dash in the middle. This clearly distinguishes the two grades and shows that the first is associated with “winter”.

What Are The SAE Tests?

The Society of Automotive Engineers is a professional body. It sets standards for many automotive projects and appliances, including motor oils.

The grade that you see on an oil is defined by the SAE.

A total of four tests are conducted on an oil. Two of these are done at 0 degrees C and the other two at engine operating temperatures.

The first two tests give the oil its winter grade. In the example of 10W30, this would be “10W”. The second two tests give the oil its operating temperature grade, which would be “30” in the same example.

Read more about the SAE grades on Wikipedia here.

10W30 vs 10W40 – Is 10W30 Better Than 10W40?

Neither oil is better or worse.

They are different from each other, certainly, and will react differently as well, but it would be a complete injustice to call one better than the other.

As we have just seen, both 10W30 and 10W40 have winter SAE oil grades of 10W. SAE grades range from 0W to 25W, and so you might think of 10W as being the sort of average “winter” viscosity across the automotive industry.

Since both oils are the same here, it won’t make any difference at this point…


Once the engine gets up to temperature, the oils behave slightly differently.

You can see that 10W40 is more viscous than 10W30, at these temperatures. The basic fact that “40” is greater than “30” tells you that.

If you were to put 10W40 in an engine designed for 10W30, the “thicker” oil would lead to friction in the engine and an increased load on the oil pump. The car would have to do more work to get the oil around the engine block.

In the same manner, putting 10W30 in an engine that wants 10W40 could lead to insufficient lubrication. The oil might be “too thin”.

The lesson here is to always listen to – and obey – your owner’s manual.

The manufacturers know what they’re talking about, contrary to what a shouty blog on the corner of the internet might be telling you.

There might be the occasional exception to this, but very rarely.

Remember – your engine was designed for use with the motor oil recommended to go with it! Changing it without warning is almost 100% certain to harm your vehicle.

10W30 vs 10W40 – What Should I Do If I Put The Wrong Motor Oil In?

The short answer is: change the oil.

It’s just not worth the risk.

If you’re working on a project car or otherwise experimenting, then a new engine oil grade might be good for your car. However, for the 99% of us who drive boring, standard, getting-you-from-A-to-B cars, it’s just best to follow your owner’s manual to the letter.

I think we’ve probably expressed the importance of that enough now.

You should change your oil immediately.

How Do You Do An Oil Change?

We have other articles that explain how to change your oil, and so won’t go into it in great detail in this one. You can read one here. Alternatively, here‘s a link to “How to Change Your Oil” from Car and Driver.

In short, you’ll need to…

  1. Have your car on level ground. Not doing this will mean the oil won’t all drain out and you’ll end up with too much when you top it back up. It might also be sludge that stays in, and that’s always bad.If the car isn’t level, some of the oil in the sump won’t drain out properly.
  2. Open the oil cap at the top of the engine and place the oil pan underneath the car, ready to catch it when it flows out.
  3. Remove the sump plug, being careful not to shear it off.
  4. Allow the oil to drain into the oil pan.
  5. After about ten minutes, once the oil flow has reduced to an occasional drip, replace the sump plug. Tighten it to the correct torque level using a mid-range torque wrench.
  6. Pour the correct amount of motor oil into the engine through the open oil cap. Make sure that this is new oil and that it’s the correct grade or multigrade. Never put old oil back into an engine. You’ll probably need a funnel of some kind for this.
  7. Replace the oil cap and tighten it until it clicks.
  8. Start the engine. For a few moments, it will clatter around and sound like your worst nightmare. After a few moments though, it should settle down. It’s just as the oil pump gets the oil sent around the engine. If, after ten or fifteen seconds, the noise doesn’t get much softer, turn off the engine and check everything. You might need some professional help.
  9. If everything has gone to plan, check the oil level on the dipstick. If it’s between the maximum and minimum marks, yay! You’re all good to go.
  10. Keep an eye on everything over the next 50-100 miles, including checking underneath the car for any oil leaks.

Should I Change The Oil Filter At The Same Time?

This is usually recommended since to change the oil filter you need to drain the oil out first.

Sometimes manufacturers are nice to mechanics and put the oil filter in places that are easy to reach. Equally, sometimes you have to take a wheel off, remove the wheel arch cover, and reach into the back of the engine with some kind of makeshift 3′-long ratchet (speaking from experience).

Oil filters are a vital part of the system. You should change it when you do an oil change if you possibly can. Not doing so could lead to problems involving particulates in the oil.


We hope this article has been useful for you and that you now know the difference between 10W30 vs 10W40.

In summary, they are the same when cold, but 10W40 is more viscous once the engine is running at its normal operating temperature.

And – as perhaps the most important thing to take away from this – believe your owner’s manual above everyone else!

Thanks for reading!

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