Why should synthetic oil never be mixed?

Synthetic oil should never be mixed primarily to avoid contamination from different brands or types. Each brand of synthetic oil may contain unique additive packages that perform specific functions, such as detergents, friction modifiers, or anti-wear agents. When these oils are mixed, the compatibility of these additives can be compromised, potentially leading to reduced performance and protection in the engine. This is particularly important because the intended benefits of each oil—like stability under high temperatures or enhanced lubricity—can be diminished when mixed.

While other concerns, such as overheating, viscosity changes, or increased wear, could theoretically arise from mixing oils, the primary and most significant reason to avoid mixing synthetic oils is the risk of contamination and diminished effectiveness of the protective additives. Ensuring that the oil in an engine is uniform and from a single source helps maintain its intended properties and performance, which is crucial for the engine's longevity and reliability.