Question: What Happens If You Mix Regular and Synthetic Oil?
Answer: Here's a practical chemistry question for you. Do you know what happens if you mix regular and synthetic motor oil? For example, 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 regular motor oil. Is it better to use the regular oil or will you risk harming your engine if you add the 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. In fact, 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.
However, 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'll 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's possible it will be displeased if the (expensive) additives can't work the way they are supposed to. This may not damage your engine, but it won't help its performance.