The simple answer: Yes. There is no danger mixing synthetic and conventional oil in your engine. However, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits.
The detailed answer: With synthetic engine oils continuing to grow in popularity, the question of whether synthetic and conventional oils can be safely mixed often arises. I am guilty of having mixed different types of engine oil with reckless abandon back in the day. A dash of synthetic blend to top-off my car one month and a shot of cheap conventional oil the next.
Although I’m sure performance suffered, my engine never blew up as a result. So, yes, you can safely mix synthetic and conventional oil. But barring an emergency, it’s not a great idea.
All engine 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 and typically contain higher-quality additives. All in all, conventional and synthetic oils are comprised of the same, compatible components – it’s just that the components in synthetics are of a considerably 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 travelling, for example, and suddenly find your engine low on oil, cheap conventional oil may be the only option at the petrol station alongside the motorway. While it’s safe to top-off an engine that normally uses AMSOIL synthetic engine 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?