Mixing Regular and Synthetic Motor Oils

Here's a practical chemistry question for you: Do you know what happens if you mix regular and synthetic motor oil?

Let's say the mechanic put synthetic oil in your car when you got your oil changed. You stop at a gas station and see you are running about a quart low, but all you can get is conventional motor oil. Is it all right to use the regular oil or will you risk harming your engine by doing so?

Mixing Motor Oil

According to Mobil Oil, it should be fine to mix oils. This manufacturer states it would be unlikely anything bad would happen, such as a gel-forming from an interaction of the chemicals (a common fear), because the oils are compatible with each other.

Many oils are a blend of natural and synthetic oils. So, if you are low on oil, don't be afraid to add a quart or two of synthetic oil if you are using regular oil or even regular oil if you are using a synthetic. You don't need to rush right out and get an oil change so you'll have "pure" oil.

Possible Negative Effects

It is not recommended to routinely mix oils because the additives in different products may interact or the oils may become destabilized by the mixture. You may reduce or negate the properties of the additives.

You could lose the benefits of the more expensive synthetic oil. So, adding regular oil to your special synthetic oil will mean you'll need to get your oil changed sooner than you would have otherwise.

If you have a high-performance engine, it may not allow the (expensive) additives to work the way they are supposed to. This may not damage your engine, but it won't help its performance.

Difference in Regular and Synthetic Oil

Both conventional and synthetic motor oils are derived from petroleum, but they can be very different products. Conventional oil is refined from crude oil. It circulates through the engine to keep it cool and prevent wear by acting as a lubricant. It helps prevent corrosion, keeps surfaces clean, and seals the engine. Synthetic oil serves the same purpose, but it's tailored for higher temperature and pressure.

Synthetic oil is also refined, but then it's distilled and purified so that it contains fewer impurities and a smaller, select set of molecules. Synthetic oil also contains additives intended to help keep an engine cleaner and protect it from damage.

The main difference between regular and synthetic oil is the temperature at which it undergoes thermal degradation. In a high-performance engine, regular oil is more apt to pick up deposits and form sludge.

Cars that run hot do better with synthetic oil. For most automobiles, the only real difference you'll see is that synthetic costs more initially but lasts longer between oil changes.