Can You Mix Different Viscosity Oils? The Details Guide

Owning a car goes with a lot of responsibilities. Sometimes, buying a car is far easier than constantly keeping one on the road. Meanwhile, changing or topping off your engine oil is one of the maintenance activities every car owner must learn.

 However, topping off the oil just like that may not be an issue. The real factor is – Can you mix different viscosity oils? This is a significant contemplation for most new car owners who are trying to top off their engine oil for the first time.

 So, if you’re refilling your engine oil, you’re wondering, “Is it bad to mix oil weights?” Carefully read this article to find out.

can you mix different viscosity oils

Can You Mix Different Viscosity Oils?

We recommend not mixing different viscosity oils but instead using the recommended engine oil. But, in emergency cases, you can mix different viscosity oils when refilling your car’s engine oil, provided the oil brands meet the American Petroleum Institute’s requirements, and the variance is not much. Mixing oil viscosities from different oils might not cause any adverse reaction in the short term. However, there are no perfect predictions in the long run.

Usually, mixing different viscosity oils may not damage your car’s engine. However, you may not get the best performance from the mixture due to the variance in viscosity. So, if you’re mixing different viscosity oils, it is essential to know that the fluids may not mix well in some cases.

Therefore, it would be better to stick to the type of oil recommended in your vehicle’s owner’s manual or consult an expert auto technician for professional advice instead of mixing oil randomly.

If you’re asking, “Is it safe to mix oil weights?” Since it is ok to mix oil weights in some cases, don’t make it a lifestyle always to flaunt professional recommendations.

Can You Mix Different Oils Of The Same Viscosity?

You can successfully mix different oils of the same viscosity. However, you may not get an efficient engine performance from the mixture. It is vital to note that mixing oil weights from different brands with varying additives can cause sludge to build up over time, thereby affecting your car’s engine performance and create engine noise.

So, the main reason you must be careful when mixing two different oil brands is because of the kind of additives used by the individual manufacturers.

If you desire the best for your vehicle’s engine, avoid mixing two different oil brands, no matter the similarity in viscosity. If you don’t have your vehicle’s recommended engine oil handy for a refill, be patient to get one instead of risking engine damage in the long run.

Can You Mix Different Types of Engine Oil?

Mixing different types of engine oils raises questions among car owners. The viscosity oils market offers many options, and the contemplation is whether a combination could cause engine rattling or damage. The answer lies in understanding the oil ingredients and their chemistry. Oils with different API values, viscosities, or grades — for diesel engines or gasoline engine oil — have distinct qualities. Motor oil manufacturers formulate each type with specific additives to meet car manufacturers’ requirements.

While in an emergency, adding a bit of a different oil won’t typically cause immediate harm, it is not advisable for long-term maintenance. Consumers should consider the risks, as mixing can compromise the integrity of the oil, potentially leading to a decrease in performance and engine protection. The consensus among scientists and industry colleagues is to follow the car owner’s manual and use the recommended oil grade for replacement during an oil change to ensure the longevity and efficiency of the vehicle.

Can You Mix 5w30 and 5w40 Synthetic Oil?

Just like combining a full synthetic oil with another semi-synthetic oil in a car, mixing 5w30 and 5w40 synthetic oil is okay. However, you may not get a completely satisfactory outcome when you mix oil weights compared to using a similar lubricant to top off the current engine oil in your car.

If you’re contemplating, “can you mix different oil weights?” Sure, you can. If you’re running low on oil and all you have handy is a 5w30 full synthetic oil, you can combine it with your current 5w40 synthetic oil. However, you may experience some performance changes in if the thickness variance is much when you choose to mix other viscosities.

Can You Mix Different Brands Of Motor Oil?

Virtually all motor oils certified by the American Petroleum Institute are most likely compatible. Therefore, you can mix a particular motor oil brand like Valvoline with another brand like Castro as long as both oils have the same viscosity.

 However, motor oil manufacturers apply different methods and technologies in their brand’s oil formulation, which may impact their products with varying physical properties.

If you’re wondering, “Can you mix different weights of motor oil?” Irrespective of the oil brand, you can mix them, provided they both satisfy the API guidelines and standards. However, sticking to your manufacturer’s recommendation is best and cool your car before adding oil.

What Happens If I Use 5w40 Instead Of 5w30?

If you use 5w40 oil in your car instead of 5w30 oil, the two oil types may mix successfully. However, you will experience a higher pressure with less flow. Also, due to the thickness of the 5w40 oil, you may experience poor fuel economy when driving.

 However, the 5w40 is more suitable for vehicles with higher mileage engines. It also delivers a better performance under warmer temperatures compared to 5w30 oil. Lastly, 5w40 is less expensive than 5w30.

Therefore, if you desire more oil flow and better gas mileage, 5w40 may not be the ideal oil for you as you may get the flip side of your expectation. Again, always stick to the oil type recommended in your vehicle manufacturer’s manual.

Meanwhile, if you’re not sure of the type of oil to use when running low on oil, kindly consult a professional auto technician to guide you on your vehicle’s recommended oil type.

What Happens If Engine Oil Is Too Thick?

If you use engine oil that’s too thick, you will most likely experience difficulty starting your car in cold weather. Also, if the vehicle eventually starts, your oil flow will be far less than when using lighter engine oil.

Thick Engine Oil

In addition, you may also struggle with bad gas mileage due to the high pressure required for the thick oil to flow within your engine to lubricate all the related components within the system.

In other words, using an engine with a very high viscosity comes with many challenges, especially if the brand isn’t the right one for your vehicle. However, the high viscous engine oil will work perfectly for cars powered by such oil type.

Meanwhile, if you’re asking, “Can you mix oil weights?” Again, it is essential to follow the guidelines and recommendations provided by your car’s manufacturer in the owner’s manual.


Sometimes, vehicle maintenance comes with issues that may require you to be discretional. Other times, you may need to consult a professional, especially when you intend to mix oil viscosity from two different brands.

So, if you were contemplating, “Can you mix different viscosity oils?” The answer has been provided above in this article.

In most cases, mixing different oil viscosity oils may be compatible in the short term. However, this practice might affect your car’s engine in the long run. So, once again, it would be best to strictly use the recommended oil type for your vehicle to avoid issues.

Solomon Osuagwu

Osuagwu Solomon is a vetted auto mechanic with over ten years of experience in the garage and five years of experience as a service writer. He prides himself in writing accurate information on professional repair guides, DIY repair guides, buyer’s guides, comparisons, and car reviews. If he’s not in his repair garage, he’s writing automotive blogs to help car owners and fellow mechanics to troubleshoot and proffer solutions to several car problems.

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