What Is The Thickest Motor Oil? (Explained!)

There are many kinds of motor oil, and their thickness differentiates these oils. So let us know which oil is the thickest. Let’s go.

So what is the thickest motor oil? The answer is a 40w-70. This is the thickest motor oil.

To know more about the thickness of motor oil, the motor oils are using a rating system. The society of automotive engineers developed this rating system or sometimes referred to as SAE. This rating will classify and differentiate the oils based on their viscosity. 

When we say viscosity, it means the resistance of the fluid to flow. Some liquids have low viscosity; one example is water. Water is a thin liquid, and it has a low viscosity. 

Other liquids like honey are known to have a high viscosity because they are thick and are not as thin as the water. The viscosity of the motor oil can also change. If motor oil is heated or cooled, there will be a change in their viscosity measurement. 

Some people are still confused with the term viscosity, but simply, it just means the thickness. If the motor oil has low viscosity, then the motor oil is not thick. 

If a motor oil has a high viscosity, then it means that the motor oil is thick. So now, let us learn more about the thickest engine oil.

What Is The Thickest Motor Oil

What Is The Thickest Engine Oil?

The thickest motor oil is the 40w-70. In terms of the motor oil’s thickness, this refers to the viscosity grade of the motor oil. If the engine oil has a higher level of number, there will be more resistance that the oil will exhibit. This is because there are additions of modern polymers; this makes the multi-weight oils familiar. 

In the case of 40w-70 oil, you should know that the number forty in there defines the engine oil’s viscosity if the motor is hot in times of operation. On the other hand, the number seventy is defining the viscosity during the operation. 

If it has a high operational viscosity, it helps in preventing the engine oil from seeping into the valve guides. It will also prevent it from the combustion chamber.

Where Do You Use 40w70?

You can use 40w-70 in racing conditions. This engine oil is also made for cars already antique with diesel engines that will excessively burn oil. Those multigrade viscosity motor oils can be able to perform in a diverse range of temperatures. 

So you have to know the usage of 40w-70 and where to use it. So you get yourself familiarized with the multi-grade viscosity motor oils.

40w70 Advantages And Disadvantages

Now, let us go to the advantages and the disadvantages of a 40w-70. It is good to know the pros and cons of 40w-70 so that you will know if you have to use it and if this engine oil is suitable and compatible with the engine that you will put it in.


There are some advantages of using 40w-70. Some products will offer the best and extreme protection and deliver high-performance and competitive engines. This kind of motor oil is also very thick, and we all know that a thicker viscosity provides maximum performance for the vehicle. 

Aside from that, 40w-70 can also help prevent oil loss and worn engines. A thicker viscosity also protects against heavy loads. And no matter what severe conditions, this engine oil can still maintain its oil pressure and viscosity. 

Using a 40w-70 can also help reduce and lessen oil burning and lessen the need to top up. It can also minimize exhaust smoke and maintain the pressure of the engine oil. So using a 40w-70 has a lot of advantages. 

However, there are some things you have to consider. First, not every vehicle is recommended for 40w-70 to put in their engine. You need to consult a mechanic first or even your owner’s manual. 

It is stated there what kind of oil grade is recommended for your vehicle. It is essential always to follow the owner’s manual and not put any other oil grade that is not recommended. This can do damage to your vehicle.


As much as there are many advantages to using a 40w-70, there are also many disadvantages. Many drivers would always make the mistake of always picking motor oil that always has a higher viscosity than recommended. They would always think that a motor with a very high viscosity level is suitable for their car. 

You must take into account that this is not always the case. Always picking a motor oil with a high viscosity level is not the way to go. It would help if you went for what is recommended to you and your vehicle. 

So the first disadvantage is that the thicker the motor oil, the more difficult for the motor oil to circulate throughout your engine. If the motor oil is having difficulty circulating your engine, this will lead to your vehicle having less fuel efficiency. This will also make your vehicle harder and challenging to start. 

If there will be a challenging time starting your vehicle, there will also be an increase in terms of wear in your engine. The disadvantage is already worse in warm weather conditions; it will worsen in cold weather conditions. 

Another disadvantage is that if a motor oil has a high viscosity level, it will not be able to transfer heat in between the components of the vehicle’s engine.

40w-70 motor oil

Should I Use Thicker Oil In A High Mileage Engine?

Not necessarily. Vehicles that have high mileage engines do not always require thicker oil. The rule of thumb is to always look for your owner’s manual and look for experts on the matter. 

It is not advisable to put on a motor oil that depends on your liking. You must consult an expert.


In summary, there is a difference in terms of the thickness of motor oil. The thickest motor oil is 40w-70. 

You must be able to know when to use 40w-70. You must refer to your owner’s manual for the instructions: the thicker the viscosity, the more complex the oil moves around in the engine.

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Image credits – Canva

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My name is Hank, and I've been in the automotive industry for 27 years. I've been working in my own auto repair shop for the last 13 years, and now I want to help you here, on my blog. Let me know if you have any questions. Read more