Can I mix old and new engine oil?

Mixing old and new engine oil can cause various damages to the engine, and it is not recommended for car owners to do so. Old oil gradually oxidizes during use, reducing the effectiveness of its antioxidants and additives while accumulating engine wear debris and moisture, leading to deteriorated lubricating performance. When mixed with new oil, it contaminates the fresh oil and accelerates its oxidation process. For instance, new oil with a potential service life of 1500 hours may only retain effectiveness for 200 hours after mixing. The blended oil promotes sludge formation, clogging filters and oil passages, impairing heat dissipation and potentially causing engine overheating. It also compromises oil film stability, exacerbating piston-to-cylinder wear and possibly resulting in piston ring fracture in severe cases. Mixing oils of different brands or specifications is particularly hazardous due to potential chemical incompatibility among additive packages, which may cause oil cloudiness, lubrication degradation, and metal component corrosion. While emergency mixing tests (observing for precipitation or abnormal odors) might be permissible under exceptional circumstances, strict avoidance of all oil mixing is essential for maintaining optimal engine performance and longevity. During oil changes, complete drainage of used oil must precede the introduction of fresh oil.