Can motor oils of different brands be mixed?
2 Answers
Motor oils of different brands can be mixed, but it is not recommended. If it's for emergency purposes, it might be acceptable. It's important to note that when mixing different oils, the viscosity grade and performance level must be the same, for example, both should be fully synthetic 5W30 or fully synthetic 5W40. The potential impacts of mixing different brand motor oils are: 1. Turbidity: Most modern motor oils use sulfonates as dispersants and detergents, while some use salicylates. These two types are not compatible, and mixing them can cause layering, turbidity, and reduced cleaning performance; 2. Abnormal exhaust: Mixing different brand oils may lead to abnormal exhaust smoke, such as black or blue smoke. The mixed oil might become diluted, allowing it to enter the combustion chamber and produce blue smoke, or it might cause poor cylinder sealing, resulting in black smoke; 3. Sludge formation: Mixing different oils can easily lead to sludge formation, which reduces the cooling effect of the lubricant, causing the engine to overheat and potentially leading to malfunctions; 4. Accelerated wear: The anti-wear properties of the mixed oil can change significantly, potentially breaking down the oil film and causing wear between the piston and cylinder wall, or even piston ring breakage in severe cases.
When I first started repairing cars, I was also curious about this issue. But after taking apart the engine, everything became clear. The additive formulas of different brands of engine oil vary too much, and mixing them can produce flocculent precipitates that clog the oil passages like an old sewer. Last year, I worked on a BMW that had mixed oils, and the filter was clogged with coffee-ground-like hard chunks. The overhaul cost the owner over 20,000 yuan. Mixing mineral oil with full synthetic oil can also cause layering, reducing lubrication effectiveness by half. The worst case I've seen was a Civic that suffered cylinder scoring after mixing oils, with deep grooves worn into the piston sidewalls. In an emergency, you can add up to 100ml of the same grade oil at most, but you must completely drain and clean the system afterward. During maintenance, make sure the mechanic doesn’t use leftover no-name oil from the oil gun—it really ruins the car.
