Can You Safely Mix Conventional and Synthetic Oil?


It’s time for another oil change. As you look in the garage, you see that you have almost enough of the nice synthetic oil you bought last time.

You also notice that you have a couple of quarts of conventional oil still on the shelf. As we all do, you want to avoid a trip to the store and wonder if you can mix the two without endangering the health of your engine.

I’m going to attempt to take the mystery out of that scenario. Let’s take a look at the effects of mixing two different oil types. Will it cost you big, or will you be fine?

Simply put, you can mix conventional and synthetic oil. When it comes down to it, all motor oils consists of base oils and a blend of additives and detergents. While some won’t be optimal for every purpose, most engines will run fine for a while using a blend.

So why even have a “synthetic” vs. “conventional” designation?

Let’s explore the differences and see for ourselves.

The Types Of Oils

The main difference between the two types of oil, conventional and synthetic, is the level of performance and the mechanical standards they should meet.

Conventional Oil

Conventional oil is primarily defined by three characteristics.

  • It meets bare minimum standards to keep an engine clean.
  • Conventional oil lasts only as long as necessary for the use case.
  • The production costs of conventional oil are as low as possible.

Since they often only meet the bare minimum for quality and performance, they are usually the cheapest oils.

Because they are the cheapest, they often contain the largest number of impurities. Pennzoil, however, makes their conventional oil from natural gas instead of petroleum. This means that the baseline number of impurities is far lower than other competitors.

Synthetic Oil

On the other hand, synthetic oils are often formulated to meet much higher standards.

They use much higher quality additives, and harder working but gentler detergents. Synthetic oils are highly refined and often contain little to no impurities. Because of this, they are far clearer than conventional oils.

The increased production costs coupled with the superior lubrication and performance means that synthetic oil carries a much heftier price tag.

Synthetic Blends

There are even oils that are a mix of synthetic and conventional, that are pre-mixed and sold as a “synthetic blend”.

This allows manufacturers to provide a mid-level product, neither bare minimum nor top-tier. Then, they sell that product at a price point that is a compromise of the two.

Exploring Engine Types

Not all internal combustion engines are created equal and different kinds often have different lubrication requirements. Let’s explore how the type of engine can have an impact on whether your should mix your oils together.

Automotive Engines

For automative engines, you can mix synthetic and conventional oil. However, this should be a temporary fix or a stopgap measure.

If you find you are a quart low, and all you can find is a quart of conventional, you should feel comfortable enough to add that to the crankcase until you can get an oil change.

As stated above, each oil has its own blend of additives, detergents, and impurities. Conventional tends to have more impurities, and this means that using it will lead to increased gunk build-up. Naturally, the opposite is true for synthetics.

With this in mind, realize that mixing conventional oil into synthetic can dilute the benefits of synthetic oil.

This cheapens the investment you initially made with the synthetic oil, by reducing the effects you paid a premium for. It also shortens the life of synthetic oil.

Additionally, mixing different oils can actually destabilize the oil on a molecular level. This can lead to reductions in efficiency, performance, and protective lubrication.

Finally, since different oil blends react differently to temperature changes, running a mix in high-performance engines can lead to oil pressure fluctuations.

Two-Cycle Engines

Conventional oil and synthetic oil can combine with gas for use in a 2 cycle engine.

The same caveats as automotive use apply about diluting the synthetic performance and potential destabilization. But as with a car engine, it would still protect the engine in a pinch.

Proper lubrication is extremely important in watercraft engines as it is on land (see our guide about winterizing boats still parked in water).

However, the majority of boat motors do not have oil sump systems. Instead, they require the oil to mix in with the gas so it can help lubricate he cylinders as they burn.

The only additional thing to think of with 2 cycle engines, is that many sources recommend using as little conventional oil as possible, so the synthetic makes up the greater proportion of the oil.

This gives the sometimes delicate 2 cycle engine a more protective blend until you can use either conventional or synthetic as the sole oil.

Captain

I'm Chuck (the Captain). I'm passionate about my outdoor space and love sharing my experiences with the world at large. I want Captain Patio to become the best place on the internet to find, share, and learn about all things patio-related. When I'm not keeping up my content schedule, I'm spending time with my wife and two kids (usually on my patio!).

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