Can You Mix 5w30 and 10w40 Oil? Answers and Advice!

A car’s engine oil is the blood flowing through the engine. Without it, the engine will not operate, and with very little, it will not run smoothly. Just like humans and blood groups, it is not just any old oil that goes in there, but a particular grade, specifically defined by the manufacturer, known for its unique properties. But, has it ever crossed your mind: can you play scientist and mix your car engine’s oil? For instance, can you mix 5W30 and 10W40 Oil?

Mixing two multigrade oils, such as 5W30 and 10W40, is ok. However, this doesn’t mean you can go about it without caution. If your engine is dangerously low on oil and all you have to hand is the opposite then top the engine up. The last thing you want to do is damage the engine because you ran out of low.

One last point before you read on, the information in this article is unique to mixing 5W30 and 10W40; not all engine oils can be mixed.

What Does 5w30 or 10w40 Mean for Engine Oil?

Just as your clothes change with the weather, you’d wear a heavy coat in winter. And for summer, you’d choose something lighter. Similarly, depending on the engine running temperatures, your engine needs different ‘coats’ or oils.

The ‘5W30′ or ’10W40’ on the engine oil container is like a code that tells you how well the oil will flow in your engine in different heat conditions. The technical term for it is viscosity. 

The first number before the letter ‘W,’ which stands for winter, indicates how easily the oil flows when it’s cold (viscosity). So, a cold ‘5W’ oil is thinner and flows faster when cold than a ’10W’ oil.

After the W, the second number tells you how thick the oil is at high temperatures, such as in a car engine. So, a ’30’ oil is thinner than a ’40’ when hot.

In simpler terms, ‘5W30′ oil is thinner and flows more easily in cold and hot conditions than ’10W40’ oil.

Multigrade vs. Single-grade oils

The oils 5W30 and 10W40 are multigrade. This means they are designed to protect the engine at low and high temperatures. The ‘multi’ in multigrade signifies the oil’s ability to work efficiently across various temperatures, making it versatile for multiple climates and engine operating conditions.

On the other hand, there’s something called single-grade or monograde oil. As the name suggests, a single-grade oil is designed to perform optimally at a specified temperature. They have a simple rating, like SAE 30 or 40, without a ‘W,’ meaning they’re best suited for constant unvarying temperatures. Most commonly, they are used on older cars or industrial applications. Although you might find them on the shelf next to your 5W30, they are not to be confused or mixed.  

Can you mix 5W30 And 10W40 oil? 

Yes, you can mix 5W30 and 10W40 Oil without causing immediate harm to your engine. However, this should be treated as an emergency measure only. 

When you mix these two oils, you’ll end up with a blend that has characteristics sitting between the two original oils. If mixed in equal parts, you create an oil with a new viscosity somewhere between 5W30 and 10W40. Depending on the sensitivity of the engine, this might not be ideal. 

What Happens if You Drive With Mixed Grades of Oil?

As the driver, you probably won’t notice anything when driving especially between these two grades specifically. 

In colder conditions, this mix would flow slightly less efficiently than pure 5W30 oil due to the influence of the 10W40 oil, which is thicker at low temperatures. This might make it trickier to start the engine at low temperatures. Similarly, the mixed oil will be slightly thinner than pure 10W40 oil under hotter conditions because of the 5W30’s influence.

The performance and fuel efficiency of an engine can be effected when driving with mixed oil grades. If one of the oils is too thick, the engine will need to work harder, which will increase your miles per gallon. There will be also be a slight dip in engine performance, but again, to be clear, you probably won’t notice this. 

If you’re considering mixing oils, check with the manufacturer or refer to your vehicle’s manual. Deviating too far from their guidelines could affect your engine’s performance or even void your warranty.

Things to Check and Consider When Mixing Two Oils

While not ideal, mixing two oils can sometimes be necessary under certain circumstances. However, this needs to be done with a certain level of precision and understanding. Here are the things you should be mindful of when blending two oils:

Brand consistency 

All oil manufacturers use specific proprietary blends of additives in their products to enhance performance, reduce friction, and prevent engine wear. 

Mixing oils of different brands can lead to unexpected reactions between these additives, which may compromise the oil’s effectiveness. If you’re considering mixing 5W30 and 10W40 oils, ensure they are from the same brand. Again don’t hesitate if you only have an alternative brand in an emergency.

Weather conditions

The numbers in oil grades (5W30 and 10W40) relate to how the oil will perform in specific temperature conditions. 

Mixing two different grades will result in a blend with a performance profile between the original oils. You need to ensure that this new blend will still be appropriate for the weather conditions in your climate. You may need to consult a mechanic if you live in an environment with extreme temperatures. 


It’s critical not to over mix the oils. The rule of thumb is to fill no more than a 50/50 ratio unless an expert or the manufacturer advises otherwise.

This might not be possible if your trying to top up an engine that is low on oil, and this is the only oil you have. In this case, don’t hesitate to top the engine up but ensure you carry out a proper oil change asap. 

Manufacturer’s recommendations

Your vehicle’s manufacturer knows the ins and outs of your engine better than anyone. They provide specific oil grade recommendations for a reason – to ensure optimal performance and longevity of your motor. 

Final Thoughts

5W30 and 10W40 oils can be mixed without causing immediate havoc to your engine. However, they come with certain conditions and caveats – equal proportions, the same brand, and consideration for the weather conditions, to name a few. If you must mix the oils, I strongly recommend you perform an oil change and refill with the correct oil at the earliest opportunity. 

Keep this information in your back pocket for those “just in case” moments, but remember – when it comes to your engine’s health, there is no substitute for the manufacturer-recommended oil.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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