Here is the scenario: You are changing the oil in your vehicle. It’s long overdue, but the month has been hectic, so you decided to do the oil change when you had a spare moment. Unfortunately, you didn’t replenish your stock of oil. All you have left is a bottle of 5W-20 and one 10W-30. This raises the question of whether you can mix different weights of oil.

While there are some discrepancies online, the general consensus to this is no, mixing different weights of motor oil is not a good idea. Now let’s explain why.

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What Oil Numbers and Names Mean

Trying to decipher the letters and numbers on the oil bottle? Those are important pieces of information. The numbers in 5W-20, for instance, explain the “weight” or “viscosity” of the oil. Weight can be slightly misleading, as it isn’t talking about how heavy the oil actually is. Both “weight” and “viscosity” refer to the thickness of the oil. The higher the end number, the thicker the engine oil.

The numbers also describe the temperature at which the oil functions. If an oil is rated 10W-30, it means that the viscosity is 10 in cold temperatures (the “W” stands for “winter”) and 30 in hot temperatures. That said, oil does not become more viscous as it gets warmer. All this means is that the oil has a weight of 10 in the winter and a weight of 30 in the summer. Generally, the hotter the oil, the less viscous it becomes.

But why is this necessary, you ask? Well, in certain vehicles you shouldn’t put a 30 weight oil in your engine. That kind of oil would be too thick, and your engine would function less efficiently. Since the 30 weight oil wouldn’t flow correctly, your engine could also be damaged in places where the too-thick oil couldn’t reach. Always check what oil your vehicle manufacturer recommends.

Similarly, in some vehicles running a true 5 weight oil is also risky. An oil that is SAE 5 would be too thin and would not protect the engine. This is why oils are formulated to have multiple grades, so the motor is fully protected in all kinds of weather.

Can You Mix Different Weights of Oil?

In the world of drag racing, it is a common practice to mix different oil weights—also known as viscosity—to come up with a unique blend all your own. Well, while that’s okay for a drag racing vehicle, it’s not okay for your little 4-cylinder sedan. While mixing together 5W-20 and 10W-30 might work out for you in the beginning, there is little evidence that this is beneficial to your car down the road.

At operating temperature, these two oils are the same. That is true. But, if you experience a cold snap, you’re going to see the difference.

Yes, there are oil grades that do mix—but only if they come from the same brand. However, you are producing a new viscosity, and it may not be compatible with your car or truck. For instance, if mixing two different weights of oil results in a heavier viscosity, your engine might have a difficult time turning over on days when the temperature is lower.

Reasons Why You Shouldn’t Mix Different Oils

Now that you know that you can mix different oil weights but should be apprehensive about it, let’s look at some other reasons why oils don’t always mix.

Additives and Formulas

Synthetic oils are made with special formulas and additives that also affect viscosity. Because different brands have their own unique solutions, there is no way to determine whether mixing two different oil weights would be effective. Since this is more or less trial-and-error, you could either help your car or hinder it. Do you really want to play Russian roulette with motor oil like that?

There is some anecdotal evidence on the internet that even switching between brands can negatively impact your engine. For instance, going to Valvoline after using Pennzoil has been known to cause oil pan gaskets to fail prematurely. Why? Because the additives in Valvoline reacted to the detergents in the Pennzoil, the gasket started to deteriorate.

Protect your engine. Use high-quality synthetic oil of the same weight viscosity and brand every single time.

New Oil Viscosity

When mixing two oil weights, the result is most often within the range of SAE 40. This is a viscosity most often used in the winter. Sure, creating a new engine oil viscosity is fine, but there will rarely be a time when you need SAE 40.

In fact, adding an oil with too high a viscosity may cause excessive heat buildup and churning loss. Your vehicle may wind up exerting too much energy while losing power, which, in turn, will burn up your fuel. Thick oil performs worse at heat transfer, too. This accelerates the oxidation of the engine and also contributes to more sludge in the engine.

In short, you shouldn’t be mixing the oils if you don’t know how it will affect the engine.

Uncertain Stability of the Mixture

Since you are mixing different motor oils together, there is a chance that they won’t play nice. This could be a result of additives disagreeing with one another, or the fact that the viscosity gets messed up. You never really know how mixing different oil weights will affect the behavior of the oil, either. For all you may know, it could make your engine sluggish or run hotter.

Therefore, if you need to change the oil or top off, it is best to use one kind of viscosity. Stick with what is recommended by the automobile’s manufacturer.

Mixing Oils With The Same Viscosity

Now, what if you have 5W-30 and 10W-30 laying around? Since the viscosity is the same, the result will be better than mixing SAE 20 and 30 together. However, this is still only acceptable if you need to top up the oil in your engine. You should not mix two different kinds of oil together during a full oil change.

Mixing Different Motor Oil Brands

Let’s say you find yourself in a situation where you need to top off the engine oil during a road trip but your favorite brand is nowhere to be found. Is mixing multiple brands of oil together safe as long as the viscosity remains the same?

Yes, it’s generally safe.

One of the greatest marketing ploys of all time is different brands of motor oil convincing people that their formula is different from the others. However, due to international regulations, most motor oil formulas have to be relatively the same, if not identical, to one another. In other words, a 5W-20 from Pennzoil is going to be the same as a 5W-20 from Valvoline.

There may be a few differences in the formula, but the oil works the same. Therefore, when you need to use another brand in a pinch, don’t sweat it so much. As long as your car has the correct oil viscosity and the correct amount, it’s going to be fine.

Now, there is one tiny caveat to this: additives. Synthetic oils typically contain additives to provide better results. Again, most additives are the same, but some companies use proprietary blends that could react poorly to the oil already in your engine.

Can Synthetic and Conventional Oil Be Mixed Together?

Conventional oil, also called regular oil, is made from crude oil and has been refined to provide adequate lubrication for the motor of your vehicle. Yet, due to the refining process, conventional oil is not as uniform in structure as synthetic oil. At the molecular level, these inconsistencies can cause conventional oil to break down more quickly and build sludge. Because of that, conventional oil needs to be changed more often than synthetic oil.

Furthermore, due to the quicker breakdown of conventional oil, you should be using only synthetic oil for high mileage vehicles.

For these reasons, you should not make mixing synthetic and conventional motor oil a habit. This is only acceptable if you need to make an emergency oil change or are running low on oil during a road trip, for example. You also shouldn’t mix synthetic and conventional oil with two different weights. This works when the oil viscosity is the same.

No harm should come to your engine if this is a short-term fix.

Keep in mind that mixing synthetic oil into your conventional oil will not make the regular oil any better. Plus, regular oil tends to dilute the synthetic formula, including the detergents, dispersants, anti-wear, and anti-corrosion additives, so many of the benefits of using a high-end synthetic are diminished. Your engine won’t have the same level of protection as it would when using only synthetic oil.

Final Thoughts

Can you mix different weights of oil? Generally, mixing oils with different viscosity ratings is not recommended. While doing it once in a while as a last resort is better than letting your engine go dry, staying with the same viscosity is safer. You can also mix synthetic and conventional oils together if the viscosity is the same. Again, this should only be done when you are in a pinch since the conventional oil will counteract any of the additives put in the synthetic oil.

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