Can Different Grades of Engine Oil Be Mixed?
3 Answers
92-octane gasoline and 95-octane gasoline cannot be mixed because their isooctane content differs. 92-octane gasoline is suitable for engines with low compression ratios, while 95-octane gasoline is designed for engines with high compression ratios. If 92-octane gasoline is added to a car that requires 95-octane gasoline, abnormal combustion may occur, leading to engine knocking. Knocking not only damages the combustion chamber but also directly affects the engine's normal operation, potentially causing severe engine damage in extreme cases. Conversely, adding 95-octane gasoline to a car that requires 92-octane gasoline won't harm the engine, but it is wasteful since 95-octane gasoline is more expensive. Differences Between 92, 95, and 98-Octane Gasoline: Standard gasoline consists of isooctane and n-heptane. Isooctane has excellent anti-knock properties, with an octane rating of 100, while n-heptane has poor anti-knock properties, prone to knocking in gasoline engines, with an octane rating of 0. If gasoline is labeled as 92, it means this grade has the same anti-knock performance as a standard gasoline blend containing 92% isooctane and 8% n-heptane. The same principle applies to 95 and 98-octane gasoline. How to Handle Adding the Wrong Gasoline: If a higher octane gasoline is added than required (e.g., adding 95-octane when 92-octane is specified), simply use up the fuel and refill with 92-octane afterward. If a lower octane gasoline is added than required (e.g., adding 92-octane when 95-octane is specified), a high anti-knock fuel additive can be used. Alternatively, the incorrect fuel can be drained and replaced with the correct 95-octane gasoline. To determine the appropriate gasoline grade for your car, refer to the vehicle's user manual or check the fuel grade indicated on the fuel tank cap.
I've been driving for many years and have seen quite a few people mix different grades of engine oil to save money or for convenience. I remember once when I was helping a friend fix his car, he mixed 5W-30 and 10W-40 oil, and after just a few days of driving, the engine started making a humming noise. Upon inspection, we found slight wear. The oil grade actually indicates viscosity characteristics—the lower the number, the smoother the flow. Mixing oils can cause unstable oil pressure and prevent even lubrication of engine components. Doing this long-term not only increases fuel consumption but also makes the engine prone to wear and even failure. While adding a small amount of a different grade in an emergency might be okay, it's really not worth the risk. It's best to get into the habit of always using the recommended oil grade from the car's manual—it's a way of caring for your vehicle. If you've added the wrong oil, replace the entire oil as soon as possible to avoid major problems from minor mistakes.
From an automotive knowledge perspective, the oil viscosity rating such as 5W-40 represents low-temperature fluidity (the 5W part) and high-temperature viscosity (the 40 part). Mixing different viscosity grades can lead to inconsistent oil quality. To draw an analogy, it's like mixing water with other liquids - when temperatures change, it's prone to separation, causing insufficient lubrication or oil pressure issues. I always check the manual's recommendations before adding oil to avoid mixing; because this may clog the filter screen, increase engine load, leading to higher fuel consumption at best, or triggering the malfunction indicator lamp at worst. In reality, a small amount of mixing might be feasible when there's no other choice, but don't make it a long-term practice. The key is to understand that engine oil's role isn't just lubrication; it also cools and cleans the engine internals, so maintaining a single viscosity grade is the reliable approach. Always choose the manufacturer-specified oil - that's the worry-free way to go.
