What happens when you mix motor oils? Risks and consequences 

It's a more common situation than you might think. The oil level drops, you don't have your go-to product on hand, and so you wonder: can different motor oils be mixed? The quick answer is yes, with important nuances. However, this doesn't mean that it's advisable.

When you combine oils of different viscosity, different base, or non-equivalent additive packages, the result can alter the behavior of the lubricant and reduce its protection capacity.

In this article, we look at what happens when you mix engine oils, what risks are involved in combining different lubricants, and what to do if you have no choice but to refill with a different oil.

Can engine oils be mixed?

From a physical standpoint, most current motor oils are mixable with each other. That is, they can be mixed without causing an immediate reaction or an instant breakdown. But it's one thing to mix oils, and quite another that they keep their performance intact. 

The problem arises because a lubricant is not just a base. It also includes an additive package designed to fulfill very specific functions, such as reducing wear, keeping the engine clean, resisting oxidation, controlling foam, or protecting aftertreatment systems.

When two different oils are combined, the end result is no longer exactly one or the other, but a blend with less predictable behavior. 

So, if you're wondering if blending engine oils is bad, the right answer would be that it doesn't always cause immediate damage, but it can reduce protection and disrupt intended performance. 

What happens if you mix engine oils of different viscosity

One of the most common scenarios is to combine products with a different SAE grade, such as mixing 5W-30 and 10W-40 oils. In this case, the main effect of the mixing is on the final viscosity of the lubricant.

Viscosity defines how cold oil flows and how it maintains the lubricating film when the engine is operating at service temperature. If a more fluid oil is combined with a more viscous one, the result is not equivalent to a new formulation developed and validated in the laboratory. An intermediate mixture is simply obtained whose behavior may not match what the engine needs.

What happens when you mix synthetic and mineral oil

Another very common question is whether you can mix synthetic and mineral oils. Technically, it can be done occasionally, but the result also means a loss of control over the lubricant's final performance.

Synthetic oils tend to offer greater thermal stability, better performance at low temperatures, and better resistance to oxidation. They are also designed to respond better on demanding engines or ones with longer shift intervals. When mixed with a mineral oil, these advantages may diminish and the end result will be less consistent.

Brand matters less than specifications

The problem is often thought to be mixing different brands. However, from a technical point of view, the essential thing is not so much the brand as the specification that the oil meets.

Two lubricants can have the same viscosity (for example, 5W-30) and still be designed for different engines. The key is whether they meet the vehicle manufacturer's requirements and the corresponding standards, such as API, ACEA, or the brand's own standards.

Consequences of mixing motor oils 

When oils are mixed very occasionally, the engine may seem to continue running as always. But that doesn't mean there's no impact.

In the short term, mixing oils can alter fluidity, cold start protection, thermal stability, and oxidation resistance. Under normal conditions, you may not notice anything right away. But if the engine works with a high load, under demanding temperatures, or during long trips, that loss of precision can become more relevant.

In the long term, regularly using mixtures of different oils can promote wear, the formation of deposits and sludge, premature aging of the lubricant, and less internal cleaning of the engine. Also, it's not always a sudden breakdown. Many times, the problem appears as a progressive deterioration that reduces the useful life of the mechanics and compromises the protection that the engine needs with every kilometer.

Therefore, there is only one situation in which mixing oil might be the best out of several bad options: an emergency. If the oil level has dropped and you do not have immediate access to the exact recommended product, refilling with a lubricant as close as possible in viscosity and specification is usually preferable to driving with an insufficient level. 

Of course, this solution must be understood as temporary and you must make a complete change as soon as possible, because the lubricant not only has to be there, but it has to respond as your engine needs, from the start to the last kilometer.

FAQs about mixing oils

Can motor oils of different brands be mixed?

You shouldn't mix different oils, but in the case of emergency it can be done as an exception, but it is not recommended. Aside from the brand, the important thing is that the viscosity and specifications of both oils are compatible with the engine. Even so, each formulation incorporates different additive packages, so mixing can alter the final performance of the lubricant.

What happens if I mix 5W30 and 10W40 oil?

When mixing a 5W30 and a 10W40 oil, you get an oil with an approximate intermediate viscosity, but not an optimized formulation. That can affect the behavior of the lubricant when the engine is cold or hot, especially if the engine requires a very specific viscosity to work properly.

Is it bad to mix synthetic and mineral oil?

It does not usually cause immediate damage if done once out of necessity, but it can reduce the performance and useful life of the synthetic oil. The blend may offer less thermal stability, poorer cold performance, and lower oxidation resistance than the originally recommended lubricant.

Is there anything wrong with refilling with another oil if the oil level is low?

In an emergency, refilling with an oil as similar as possible to your current oil is usually better than driving with the level below the minimum. However, it's also advisable to check that the added product meets the appropriate specifications and, if you've mixed large amounts, perform a complete oil change.

How do I know which oil to use in my car?

The best place to look is always the manufacturer's manual. It indicates the appropriate viscosity and the specific approvals that the lubricant must meet. You can also search by model or license plate in a motor oil finder to verify the correct option.