Can You Mix Oil Viscosities? 5 Important Things You Should Know 

When selecting the best engine oil, aside from brand, the viscosity or oil grade is often the primary deciding factor. Although all vehicles have specified the type of oil for their engine, drivers still occasionally have to mix engine oil.

Mix Oil Viscosities

In some circumstances, we are faced with the question of mixing different oil brands or synthetic with semi-synthetic oil. We’ve discussed in our previous articles substituting multi-grade oil with each other – mixing 5W30 and 5W40 or using 10W30 instead of 5W30, among others.

So this article seeks to clear all uncertainties and answer all your questions on mixing motor oil viscosities. So keep reading!

1. Can You Mix Oil Viscosities?

Yes, mixing engine oil with different viscosities is generally safe but should only be done as a last resort. The numbers or oil grades indicate how the oil flows under specific temperatures. When you mix engine oil with different viscosities, it can result in an unpredictable impact on your engine’s performance.

Read more:

Can You Mix 5W20 and 10W30 Motor Oil? Yes, But Read This First

Can I Mix 5w30 And 10w40 Oil? Yes But Not Recommended

Can You Mix 5w30 And 5w40 Oil? Yes But Not Ideal

Since the lubricating oil complies with the American Petroleum Institute (API) there is the potential to mix engine oils if the standard is met.

There is clear evidence of the potential to mix different oil viscosities so that it does not have any detrimental effects on the engine. This refers to a situation where you are stranded and have no other way to add an identical oil type and viscosity to your car. 

Mixing engine oil with different viscosity is a better option than running low in oil or empty, which can have a detrimental effect on the engine.

Still, the rest of the operation depends on the difference between the two oils you have mixed. There is no way to predict the impact of mixed oil, so ideally, you should only mix the closest oil grade or mix to top up when you are low in oil.

2. How to Measure Engine Oil Viscosity?

The Society of Automotive Engineers created a scale for engine and transmission oils. The standard classification “XW-XX” is used to indicate viscosity. 

The viscosity (flow) of the oil is rated at zero degrees Fahrenheit by the number before the “W” (winter) (-17.8 degrees Celsius). 

The oil thickens less in cold temperatures, so the lower the number. The digits after the “XW” stand for the oil’s resistance to thinning at high temperatures and denote viscosity at 100 degrees Celsius.

Here’s a very informative video on understanding the numbers in motor oil products:

The engine will gain from using oil with low viscosity throughout the winter and for vehicles in colder climates. It will perform better in the summer and hotter climates by using oil with higher viscosity at 100 degrees Celsius. 

It’s crucial to consider the environment the automobile will be used in when comparing lubricants. The engine will start more quickly in the winter with thin oils less likely to thicken in the cold and run more efficiently in the summer with thick oils less likely to thin in the heat.

3. Why is oil viscosity important?

The viscosity of engine oil affects the operation of your cars, machinery, and equipment in a variety of ways. 

For starters, it influences film thickness. One of the primary functions of motor oil is to provide a thick coating that separates two metal surfaces and inhibits metal-on-metal contact. 

Oil viscosity increases due to high pressures, allowing full-film separation even under harsh operating circumstances. Viscosity also influences film strength, which allows the oil to reduce friction and wear. 

The film strength rises at very high temperatures and loads. Through increasing film thickness and strength, oil viscosity decreases machine susceptibility to particle contamination and wear rate.

Furthermore, the viscosity of motor oils influences:

  1. Lubricant life expectancy: Friction affects lubricant life. 
  2. The oil’s fluidity: ensuring that it reaches the frictional engine elements. 
  3. Machine and vehicle operation at high temperatures: improving chemical stability and part movement.
  4. Energy consumption: minimizing mechanical friction leads to increased energy consumption.

4. What does viscosity have to do with engine protection?

The most significant characteristic of a lubricant is its viscosity. The oil’s ability to respond to temperature, pressure or speed variations impacts how well it protects your car. 

Lubricants with low viscosity for your engine may cause the following issues: 

  1. Metal-to-metal contact and wear have increased. 
  2. Fluid friction has increased, lowering fuel economy. 
  3. Insufficient cold-temperature start 
  4. Lubricants with excessive viscosity may potentially harm your engine by causing corrosion. 
  5. Increased oil consumption results in hazardous deposits and frequent top-offs. 
  6. Increased operating temperatures expedite the degradation of oil. 
  7. Water-leaking seals 

5. Why do different vehicles require different viscosities?

Since the engines in each car model differ in speed, temperature, and load capacity, the engine oil viscosity necessary for each kind of engine varies. An excessive operation may result from using engine oil with a viscosity higher than that advised in the instructions.

A sufficient oil film may not be created if an engine oil with a thickness lower than the required number is used. As a result, there may be wear or scrapes inside the engine, leading to fractures.


Whether you own a classic car, truck, boat, or motorcycle, engine oil plays an important role in the performance and life of your machine. You don’t want to wait until the oil gets dirty or you may end up with irreversible damage.

The general rule of thumb is not to mix oils of vastly different viscosity, but it is still possible. 

Ultimately, the process of mixing engine oils should be reserved for emergencies only. But, if all else fails, you can rest easy knowing that it won’t damage your engines.

Elevate your car maintenance game by reading our comprehensive guide on engine oil. Learn everything from the basics of oil types, to how to properly maintain and change it. Keep your vehicle running at its best, read our guide now!

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