Is It OK to Mix Oil Brands?

by Synthetic Oil | Last Updated: October 7, 2022

One of the most asked questions when it comes to DIYing an oil change is whether you can mix various engine oils together. Some people want to know if you can mix different oil weights or viscosities. Others are curious if conventional oils can be mixed with synthetic ones. However, the next question on your list should be whether different oils with the same weight can be blended. The answer may surprise you.

So is it OK to mix oil brands? Let’s find out.

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Can You Mix Different Oil Brands?

Generally, you might think it is safe to mix two oil brands —so long as they are the same kind of oil. After all, what could be so different about two brands of synthetic oil? Pennzoil, Valvoline, and Castrol all contain the same stuff, right?

You are not wrong. API oils are supposed to be compatible and have the same base ingredients and additives.

However, manufacturers cannot simply copy one another. Thus, they make up their own technologies and procedures for their synthetic blends and full synthetics. Even if they meet the API guidelines, it does not mean those oils are perfectly mixable.

Take AMSOIL, for example. Even within their lineup, their formulas are completely different. Within the Signature Series, they use a blend of 2 or more oils for the base and have a specialized additive package for that product. The formula for the OE Series is completely different. Thus, if you tried to mix those two together, the oil formulations will no longer be synergistic.

By mixing two brands, you get something else, something that is neither Brand A nor Brand B, but a blend of the two that is not necessarily better. In short, you have made something unpredictable, and unpredictability when it comes to your engine is never a smart move.

What Happens if You Mix Oil Brands?

In the long run, mixing Mobil engine oil with STP or ACDelco can be problematic. And this is true for any engine oil brand. Different technologies, antioxidants, and detergents may all have the same effect—but not when mixed. As mentioned earlier, companies have their own unique formulas for their products. Each formula is designed to work with those ingredients. Add in other ingredients, and the teamwork starts to break down.

The anti-wear, antioxidant, and detergent additives cannot do their job properly. The lifespan of the oil can also be affected.

But will it impact your engine’s longevity or performance? That depends on the brands and how they blend together. If you mix two different brands with the same viscosity, you probably won’t experience any immediate issues.

That said, it is best to stick with the same brand when you have the option. You simply do not know when two brands are going to interact poorly and mess with your engine’s performance.

Check out this point of view from a mechanic:

Can Different Synthetic Oil Brands Be Mixed?

In most cases, synthetic oils can be mixed so long as they are the same viscosity and purpose. On most bottles, the manufacturers state that you can safely switch between brands or combine them.

Just be mindful of the additives!

Do Additives Change Anything?

Is it OK to mix oil brands even when the additives are different? Let’s head back to the chemistry table for a moment. In the automotive industry, additives are a buzzword. Companies are continuously attempting to see which additive is the best, so they can add it to their formula. Thus, you get a bunch of oil companies adding proprietary additives to their synthetic oil in hopes that it will be better than the competition.

So while the base ingredients are the same, the additives in each formula are very different. Plus, you cannot count on a company to tell you every additive in their oil. It’s top-secret information.

Thus, you do not really know what happens when these additives mix. The good news is that most additives are often similar enough to play nice when blended. Then there is the downside: some additives will react to one another. For example, some additives in Pennzoil react poorly to those in Valvoline and can cause damage to the inside of the motor. Some people have reported faulty gaskets!

It may be unlikely that this happens to you, but it is still possible and preventable.

Is It OK to Mix Oil Brands?

If you need to mix engine oil brands when topping off before your next oil change, it is possible. However, mixing oils is never recommended. Different brands have proprietary blends of additives that can interact with those in other brands, causing adverse reactions within your engine. Sometimes you may not have the choice about the oil you use, but it is highly recommended that you get an oil change as soon as possible to prevent any damage.