Mixing engine oil viscosities. Is it possible?

The engine viscosity is the first important characteristic when choosing the right oil for your engine. Opting for the best oil viscosity for your car ensures the engine a reliable run over time, minimal mechanical friction, good protection for its parts, and good fuel consumption for your car.

Engine oil viscosities can be kinematic, dynamic, conditional, and specific. However, most often, kinematic and dynamic viscosity indicators are used to select a particular oil. To choose the optimal lubricant, you need to carefully understand engine oil viscosity.

Sometimes, drivers face the situation when they need to add an engine oil with a different viscosity in their car. So the most common question that is always asked is:

Can I mix different engine oil viscosities?

The quick answer to this question is yes, you can mix different engine oil viscosities, but you have to consider certain factors when you do. All modern engine oils can be mixed whether if it’s fully synthetic, semi-synthetic, or mineral. This will not cause any negative impact, such as chemical reaction the crankcase, foaming, bubbling. However, this is not true for all engine oil types.

In some documents, there is clear evidence written in the safety requirements of the possibility of mixing different oil viscosities in such a way that it does not cause any destructive consequences for the engine.

All car oils have a certain standardization according to API (American standard) and ACEA (European standard) (read more here), and since the lubricating oil meets these standards, in this case, it does not matter which class, this requirement is met to have the possibility of mixing engine oils.

To provide a better answer to the question of whether different types of oil viscosities can be mixed, we will say that this operation should be done as a last resort.

And by this I mean if you are stuck in a place where you have no other options to add an identical oil type and viscosity in your car, maybe when you’re on a trip, far away from a gas station or an automotive shop, or on a highway.

In this emergency situation, you can add up the only available oil option in the engine. But, the further operation depends on the difference between the oil you have in the engine and the added one.

Let’s take for example two most used engine oils, 5W30 and 5w40. Their viscosities are very close, and even more so if the manufacturer and their class are the same, then mixing these two types of oil is possible and even driving like this until the next change. Furthermore, it is allowed to mix the neighboring dynamic viscosity values ​​of 5W-40 with 10W-40.

As a result, you will get a certain average value, which depends on the proportions of both compositions (in this case, you will get a certain composition with a conditional dynamic viscosity of 7.5W-40, subject to mixing the same volumes).

Mixing similar viscosity values, but which belong to other classes, is also allowed for long-term operation. Here I’m referring to mixing fully synthetic and semi-synthetic engine oil which is allowed or mixing a semi-synthetic oil with a mineral one, which is also allowed under these circumstances.

However, the only mix that is not that ok is between mineral oil and fully synthetic oil. You can mix these oils only if there is no other option and if you do, change your engine oil when you get to the nearest workshop.

When you have different oil viscosities but from the same manufacturer especially a well-known brand, and, if the oils are similar in both viscosity and quality, including API and ACEA standards, you can safely mix them.

However, you will have to pay attention to the car manufacturer’s tolerances. For some car make and models, their manufacturer explicitly states that the oil must necessarily meet the tolerance. If the added oil does not have such a tolerance, then you can not drive with this mixture for a long time. You need to replace it as quickly as possible and fill in with the required oil that has the necessary tolerance.

Sometimes there are situations when you need to fill in oil when you’re on the road, you drive to the nearest car dealership or automotive shop. But its range does not have the same oil you have. What to do in this case? 

The answer is simple – fill the same or better. For example, you are using 5W-40 semi-synthetics. In this case, it is advisable to pick up 5W-30. However, here you need to be guided by the same considerations given above. Oils should not differ greatly from each other in characteristics. Otherwise, you’ll need to replace the mixture as soon as possible with the right required oil for your engine.

What viscosity does a fully synthetic, semi-synthetic, and mineral engine oil has?

A widespread misconception is that fully synthetic engine oil has the best viscosity, while mineral oils have the poorest viscosity. This is not true, because mineral oils are practically thicker being found in shops with viscosity values such as 10W-40, 15W-40, and so on. So, basically, there are no low-viscosity mineral oils.

Fully synthetic, and semi-synthetic engine oils are another matter. Modern chemical additives that are used in these types of oils achieve a decrease in viscosity, which is why oils, for example, with the popular viscosity of 5W-30, can be both synthetic and semi-synthetic. So, when choosing an oil, you need to pay attention not only to the viscosity value but also to the type of oil.

Manufacturers offer a wide range of oils with the same viscosity but with different classes. Therefore, when buying particular engine oil, the choice of its type is a separate issue that must be considered based on the condition of the engine, such as the car make and model, oil cost, and so on.

The viscosities values have the same SAE designation. But the stability and durability of the protective film will be different for different oil classes.

So it should be noted that the thickness of the protective oil film formed does not depend on its strength. So, the mineral oil film can withstand a load of about 900 kg per square centimeter, and the same film formed by modern fully synthetic oils can already withstand a load of 2200 kg per square centimeter. And this is with the same oil viscosity.

Conclusion

Mixing different oil viscosities is somehow allowed, with some mixings you can drive for a long time, and with others only to the nearest car workshop to completely change your oil and search for the reason why the engine has this high fuel consumption.

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