Q: Can conventional and synthetic oil be safely mixed?
A: With synthetic lubricants continuing to grow in popularity, the question of whether synthetics and conventional oils can be safely mixed often arises. Yes, you can safely mix synthetic and conventional oil.
All motor oil, whether synthetic or conventional, is a combination of base oils and additives. Each are designed to be used in the same applications, so they each pass the same minimum standards. The main difference is conventional oil is typically designed to just barely meet standards while reducing the cost as much as possible. Synthetics, on the other hand, usually surpass the minimum standards, and they typically contain higher-quality additives, too. In the end, conventional and synthetic oils are completely compatible. However while you can safely mix synthetic and conventional oils, you’re diluting the performance of the synthetic oil. In fact many companies premix them, calling them synthetic blends, and sell them as a mid-grade performance product.
Information provided by: Amsoil
