The simple answer: Yes. There is no danger mixing synthetic and conventional motor oil; however, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits.
So, yes, you can safely mix synthetic and conventional oil. But barring an emergency, it’s not a great idea.
All motor oil, whether synthetic or conventional, is a combination of base oils and additives. Synthetic base oils are manufactured using a process that removes the impurities inherent to conventional base oils. Synthetics typically contain higher-quality additives, too. So, in the end, conventional and synthetic oils are comprised of the same, compatible components – it’s just that the components in synthetics are much higher quality. While you can safely mix synthetic and conventional oils, you’re doing nothing more than diluting the performance of the synthetic oil.
You may not have a choice in some cases. If you’re traveling, for example, and suddenly find your engine low on oil, cheap conventional oil may be the only option at the gas station alongside the highway. While it’s safe to top-off an engine that normally uses AMSOIL synthetic motor oil with conventional oil, we recommend changing the oil at your earliest convenience. In fact, AMSOIL does not support extended drain intervals where oils have been mixed.
When you’re using the best oil available, why add anything to it?