Can You Mix Oil Weights?

As a car owner, knowing car maintenance basics is essential for the long-term sustainability of your vehicle. One important maintenance aspect of the car that people often get confused about is engine oil, also known as motor oil. It provides the necessary lubrication to your car’s engine.

The importance of motor oil can’t be undermined for the health of your car’s engine. It helps maintain the robustness of your car’s internals and improves overall efficiency. While changing your car’s motor oil, the question of mixing oils of different weights must have struck your mind. Let’s explore this very question and figure out whether you can mix oil weights.

Can You Mix Different Oil Weights Or Grades?

The motor oil you use in your car comes in the form of different weights, types, and brands. Therefore, it’s not unnatural to ask yourself whether mixing oils having varying weights can affect the performance of your car. Well, the quick answer is that, yes, you can mix motor oil weights without degrading your car’s performance in the short term.

However, to understand the topic of mixing oil weights better, we have to dissect this question and start with the basics of engine oil. Keep in mind that terms like “viscosity”, “grade”, and “weight” are interchangeable when it comes to motor oil and mean the same thing.

What Do We Mean By Different Oil Weights?

Before we dive deep into the topic of mixing oil weights, let’s first build a solid understanding of motor oil weights. You must have seen numbers like 5W-30 and 10W-40 on the packaging of your motor oil. How to interpret them?

Well, such kinds of oils are known as multi-grade oils as they come in different viscosities. The first number represents their weight or viscosity during colder temperatures when the car is stationary. At the same time, the second number is the lesser viscosity that the oil takes while the engine of the car is operational.

Hence, a 5W-30 oil has a more viscous weight of 5-factor during colder temperatures but acts as a thinner, less viscous oil of 30-factor when the car is running. The “W” in this tag stands for winter. Similarly, for a 10W-40 oil, 10 would mean the stationary viscosity factory, while 40 represents the running viscosity of the oil at a higher temperature.

The reason people prefer multi-grade oils over single-grade oil is straightforward. Its purpose is to compensate for the varying temperature in the surrounding, which could affect the engine’s performance, especially during the winter. Now that you understand how oil weights work let’s refer back to our original question. Can you mix oil weights?

Can You Mix 5W-30 Oil With 10W-30?

When it comes to 5W-30 and 10W-30 oil grades, both have varying weights during colder temperatures but gain the same viscosity during hot temperatures when the engine is operational. Hence, mixing the two is completely fine and wouldn’t affect the performance of your car in any negative way. 

Therefore, you can mix them, especially when you are topping up your car’s engine with one type of oil while the other oil grade is already present in the engine.

Does Mixing 5W-30 Oil With 10W-30 Enhance The Car’s Fuel Efficiency?

We must address the misconception that mixing a 5W-30 with a 10W-30 would enhance the car’s fuel efficiency. That is certainly not the case. On the contrary, making a habit of mixing these two grades can negatively affect your car’s engine in the long term if the viscosity of the oil in your engine is constantly changing. Using oils of viscosity different from your engine’s requirement can reduce the engine’s life.

Can You Mix 5W-30 Oil With 10W-40?

Answering this question can be a little trickier as the two oil grades have varying weights in cold and hot conditions. Hence, adding them up would create two entirely new viscosities of the mixture for stationary and hot temperatures. Under certain conditions, such a practice should be completely fine.

Many car engines are designed to be compatible with a range of oil weights. Say your car engine can work with motor oils from 30 to 40 viscosity; then mixing a 5W-30 with 10W-40 should be completely fine as the mixture would have its operational viscosity somewhere between the 30 and 40 factors. Make sure you refer to your car’s manufacturer’s manual before using a mixture of oil.

Can You Mix Two Different Brands Together?

So far, we have discussed that mixing oil weights doesn’t harm your engine in the short run unless you make a hobby out of it. However, can you mix motor oil of two different brands? Generally, it’s not a recommended practice, and depending on the brands used, it can drastically harm your motor engine.

You must remember that different brands have varying chemical compositions and components in their oil. Many car engines are designed to work for certain brands with specific chemicals in them. 

That said, mixing two different brands is not the biggest sin under the sun if you have to mix different brands during the oil top-up of your car, but for the long-term sustainability of your car, you should stick to your manufacturer’s recommended brand.

Long-Term Effects Of Mixing Oil Weights

If you keep mixing varying oil weights and brands over a long time, you can end up degrading your vehicle’s performance, decreasing its health and efficiency. When you use oil grades either thicker or thinner than your car manufacturer’s recommendation, you can harm the engine in various ways.

Oil grades thicker than what’s recommended for your car may lead to the following:

  • Decrease in fuel economy.
  • Shorter lifespan of the engine.
  • Increased load on the car engine.

Similarly, oil grades thinner than what your car needs may affect it in various ways:

  • Excessive wear and tear in the engine.
  • The decreased lifespan of the engine.

Always ensure you follow the manufacturer’s recommendation when dealing with such sensitive components of your car.


This concludes our guide on mixing motor oil weights. As you have seen, mixing different weights of motor oil is acceptable, but only to a certain degree, and shouldn’t become a permanent habit. In any case, you should always refer to your engine manual before any experimentation, as it might be dangerous to its health.






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