10w30 vs 10w40 – can i Use 10w40 Instead Of 10w30?

You need to change the car oil as part of its maintenance. Sadly, the oil change of your car is problematic and not hassle-free. And it gets even worse when you don’t know what oil to use for the car. That’s why most people are confused between 10w40 and 10w30 oils. so Can I Use 10w40 Instead Of 10w30?

In general, 10w40 is a thicker oil and ideal for use in hot weather. On the contrary, 10w30 is slightly better suitable for colder temperatures. Also, rarely any car model will allow you to use both oils interchangeably unless it is special. But, in reality, many car owners breach the recommendation and use 10w40 instead of 10w30, especially in hot temperatures.

The answer may seem pretty confusing. So, to understand it thoroughly, you have to know the chemical formula of these two oils and how they work in different conditions. And in today’s article, we will do the same.

10w30 vs 10w40 - can i Use 10w40 Instead Of 10w30?

Understand the Relation between Oil Viscosity and Temperature

Before you want to know which oil is better for your car between 10w30 and 10w40, you have to understand the meaning of oil viscosity. Also, you need to know the exact relation of the oil uses with the ambient temperature.

Oil viscosity refers to the thickness of the oil, just like any liquid material. Also, viscosity defines the flow rate and mixing ability of different liquids. The viscosity of any liquid is measured, with water being the standard.

For instance, oil is thicker than regular water. So, it has a higher viscosity level. It means when you pour water and oil simultaneously, you will see water flowing faster compared to oil.

When it comes to different grades of motor oils, both their viscosity and ambient temperature comes into play. It is because the ambient temperature affects the thickness of the oil, aka viscosity, directly. As the temperature shrinks, the motor oil becomes thicker.

You can prove it with a simple trick. Put the motor oil in the refrigerator for several days. You will see it transforming into a wax-like substance. It proves that ambient temperature and viscosity of the oil are dependent on each other.

The relation works like this-

  • Oil viscosity increases as the temperature reduce
  • Oil viscosity or thickness decreases when the temperature increases

So, the relation between temperature and motor oil viscosity is proportional. And that’s where finding the right viscosity of motor oil is important as the engine will fail when the oil viscosity is inappropriate or inaccurate.

Since the oil motor needs to adapt tothe changing temperature of the engine and environment, the viscosity keeps changing continuously. Thus, many oil grades define the oil’s viscosity and show how it can cope with the changing temperature.

In this regard, 10w40 and 10w30 oil grades are the two most common motor oils. You need to understand their viscosity and working temperature to know their different applications. Only then may you know whether or not they are interchangeable.

difference between 10w40 and 10w30 engine oil

Before we head to answer this question, let’s see the difference between these two oil grades. Yes, we understand that their numbering makes it difficult for you to differentiate between the oil grades; we will make it easier for you.

Of course, the first part of the naming signals that these two motor oils are similar. But their second part suggests that they are different. So, what do these two various parts of the naming mean?

We will begin with the similarities. In cold water, they have similar performance. In colder temperatures, both motor oils have similar flow rates. So, they provide the same level of protection to your car’s engine in cold temperatures.

But their performance changes in hot temperature. But before knowing their performance in hot weather, let’s clarify their viscosity or thickness. It will help you better understand their performance and protection under certain conditions.

As you may guess, 10w40 motor oil is thicker than the 10w30 motor oil at the same temperature. Usually, an oil motor’s thickness is defined as heaviness. Thus, you may refer to 10w40 motor oil is thicker than its counterpart 10w30.

Now, let’s talk about the performance in hot temperatures. Since 10w40 graded motor oil is thicker and has a higher viscosity level, it maintains a better flow rate compared to 10w30 motor oil. Improved thickness and flow rate ensure better protection of the engine parts from abrasion and damages for a longer period.

The meaning of the numbers and letters in the motor oil:

Both 10w40 and 10w30 are multi-grade motor oil. The different numbers and letters used in their naming suggest the following things-

  1. The first letter ‘10’ refers to the oil viscosity in cold temperature from 0°F to 32°F mostly. The number may vary from 0, 5, 10, or 20. It relates to the thickness of oil in low weather.
  2. The letter ‘w’ refers to winter temperature and is related to the first part of the naming. It doesn’t have any practical value.
  3. The second or later part of the naming, “30′ and “40,” refers to the motor oil viscosity or thickness in hot temperatures. The lesser the number, the less thick the motor oil will be. So, 10w30 is less thick compared to 10w40 in hot temperatures.

Can I Use 10w40 Instead Of 10w30?

So, now let’s talk about the all-important question. Can you interchange between these two different motor oils for your car engine maintenance?

At first glance, you may be fooled to think that these motor oils are easily replaceable as their main difference lies only in the viscosity and thickness. But, the reality is quite the opposite.

Most car engines are made to work differently, and the user manual doesn’t recommend you to interchange the two oil types.

The application of 10w30 and 10w40 multi-grade motor oil will depend on the surrounding temperature mostly.

When You Should Use 10w40 motor oil?

Auto Mechanic holding 10w40 motor oil

At higher temperatures, 10w40 motor oil has higher viscosity and thickness. Thanks to such increased thickness, 10w40 motor oil is preferable for automobiles in tropical regions and summer.

Since 10w40 has better viscosity, it is less likely to get thin in hot weather. So, it protects the engine parts from metal corrosion and damages better. It does the lubrication with its added thickness, and so, most car engines have 1-w40 motor oil.

When You Should Use 10w30 motor oil?

As you know, 10w30 motor oil has less viscosity. So, it will not thicken as quickly as 10w40. Thus, in cold regions, experts recommend using 10w30 motor oil for engines to better protect them from corrosion and heat.

In cold temperatures, the effect of the external environment is almost zero. So, the lubricator actually needs to deal with the engine temperature only. It makes maintenance easy. So, 10w30 oil performs quite well even with less thickness.

Which Oil Is Better For Mileage? (10w30 vs 10w40)

For an older car with better mileage, experts suggest using thicker or higher viscosity oil.

It means, if your car’s user manual permits, you should use a thicker lubricator for the car’s engine to get extra mileage. Therefore, it is recommended that you should use 10w40 motor oil for older cars.

As older oil may have already gone through massive wear and tear, the thicker 10w40 will protect the engine’s part from further corrosion and damages. It also helps the engine handle higher temperatures better. Thus, the engine’s efficiency rise, and the car get a bit of extra mileage.

But the selection isn’t easy.

First, you must read the user manual to know whether or not the engine may accept 10w40 oil. If the manual says that 10w40 is acceptable motor oil, you are good to use it.

However, many manufacturers don’t strictly speak about which motor oil or lubricator you should choose for the engine. In such cases, you can consult the car maintenance and repair workshops to know the acceptability of a specific lubricator for the engine.  

Read also >> Can I Use 10w40 Instead Of 5w30? (Quick Answer!)

Read also >> 0w30 Vs 0w40 Motor Oil (4 Key Differences Explained!)

Final Words

Although theoretically, you can interchange these two lubricators or motor oils, practically, it is not recommended. Car manufacturers, in most cases, will accept specific motor oil for their engines. They analyze the car’s selling locations and prepare the engine to adapt to the conditions.

Nonetheless, if the car engine accepts, in cold weather, you can choose 10w30 motor oil. And for hot temperatures during summertime and tropical regions, prefer using 10w40 motor oil. It protects the metal parts of the engine from corrosion and damages to extend its lifespan.


Who Worked on This?



I'm the guy responsible for ensuring that every blog post we publish is helpful for our reader.

Mahir Ahmed


I'm the guy responsible for ensuring honest, informative, accurate and helpful guide to the reader.

Leave a Comment