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

In this article, we will discuss the common question “Can you mix 5w30 and 5w40 oil? Just like mixing oil brands, there are many discussions out there that say it’s not safe. So we’re going to set the record straight once again!

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In our previous article on mixing fully synthetic and semi-synthetic oils, we stated that, although not recommended it is perfectly safe to mix both types of oil. This is especially true if you have no other choice and don’t want to risk dirty or low-level oil.

However, when the topic comes to mixing 5w30 and 5w40 oils, some people seem to disagree with us on this point. So let’s take a look at why it’s fine to mix these oils and put any concerns you may have aside!

What’s the Difference Between 5w30 and 5w40?


Imagine this scenario, you are out of town and the low oil volume warning has come on. Your car currently has Castrol Edge 5w-30 and the only oil you can find locally is a Castrol Edge 5w-40. Is it ok to mix the two?

First, we have to discuss the difference between 5w30 and 5w40 engine oils. The numbers represent the viscosity of multi-grade oils and the W stands for WINTER. Let’s take, for example, the 5w30.

The first number, which is 5, is the weight that the oil emulates in cold temperatures. The last number 40, is the oil weight under the normal operating temperature of the engine. Thus a 5w30 oil is a 30-weight oil that acts like a 5-weight oil in the winter. While the 5w40 is a 40-weight oil with similar weight in the winter.

Related: Can I Use 10w30 Instead of 5w30? 10w30 vs 5w30 Engine Oil

Can You Mix 5w30 And 5w40 Oil?

Now that we have defined their differences, let’s answer the abovementioned question. First of all, good thing, he found a Castrol Edge 5w40. Because generally, the answer is yes. Several viscosity grades are compatible and possible to mix, especially when you are low in oil. It won’t cause any adverse effect on your engine. 

However, as much as possible, use the same motor oil brand. Moreover, mixing the two will produce a new viscosity that is not the same as intended by the manufacturer so it’s best to change to the correct viscosity as soon as possible.

What will happen if you mix 5w30 and 5w40

We can never be certain of how stable the mixture of two oil weights will be. But for your peace of mind, if you are already low on engine oil, a mixture of 5w30 with 5w40 can still do the job. It can still provide proper viscosity and lubrication to the internal parts of your engine even if it’s not ideal. 

To be on the safe side of things, you are better off buying the correct oil weight for your car which is already the end product of a mixing process.

But if you are a car enthusiast and would like to experiment with mixing different weight oils, that would be a bad idea. “But you just said that you can mix the two?”. Yes, we said that, but for emergency or top-off purposes only.

Related: Can you mix 5w20 and 5w30 engine oil? Read First

Motor oils are the product of precise recipes. Mixing the oil weights would change that ratio and could create an unstable or inferior product for your car. Moreover, mixing oils of different brands could also affect performance in a negative way. Let’s talk about these two factors:

  1. Additive Formula and Chemistry – Every motor oil brand has a special formula with different additives for a specific purpose. These additives are there to maintain the cleanliness, viscosity, and efficiency of your car’s engine. Mixing brands will change some of these properties that may unpredictably affect performance.
  2. Uncertainty of Oil Mixture Stability – As we’ve mentioned earlier, the mixture of different weight oils will not produce the same viscosity as advertised. Mixing intentionally and in the great ratio (50/50), would create a new viscosity in between that is either too thin or too thick to lubricate your parts effectively.

Engine oil is probably the most crucial aspect of vehicle maintenance you should keep your eye on. If you want to add one weight of oil to another, it’s better if it has the same brand name. That will ensure there are no oil additives that will change and is already in the perfect ratio.


So, can you mix 5W30 and 5W40 oil? Just like mixing full synthetic with semi-synthetic oil – mixing different viscosity such as the 5w30 and 5w40 is not a bad idea, especially if you are running low on oil. However, doing so is not ideal if you want to reap the benefits of what the motor oil brand intends. Just like mixing oil brands would change things that could affect performance, it’s always better to be on the safe side and use what was intended for your car in the first place.

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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