Is it okay to mix 5w30 and 5w40?
3 Answers
Different grades of engine oil have varying performance characteristics. For performance-oriented vehicles, 5w30 and 5w40 should not be mixed. For less demanding vehicles, mixing a small amount may not cause issues, but they should not be mixed in equal proportions, used long-term, or substituted directly. If a 5w40-specified engine is filled with 5w30, it may lead to oil burning. Therefore, avoid mixing unless in emergencies, as it could potentially harm the engine. Difference between 5w30 and 5w40: 5w30 and 5w40 indicate the viscosity of the oil, which is suitable for both winter and summer use. The 'W' stands for 'Winter.' The number before 'W' represents the low-temperature viscosity—the smaller the number, the lower the low-temperature viscosity and the better the low-temperature fluidity, making it suitable for colder climates. The number after 'W' represents the oil's viscosity at 100°C—the larger the number, the higher the viscosity and the better the high-temperature protection. Recommended engine oil for different car models: Different manufacturers have varying technical requirements for engines and thus different viscosity requirements for engine oil. Japanese and Korean models typically recommend oils with viscosity grades of 20 or 30, such as SAE 0W-20 or 5W-30. American models generally recommend oils with a viscosity grade of 30, such as SAE 5W-30. European models usually recommend oils with a viscosity grade of 40, such as SAE 5W-40. If unsure, car owners can refer directly to the recommended oil type in their vehicle's maintenance manual.
As a veteran driver with decades of experience, I must say mixing 5w30 and 5w40 engine oil is really not a good idea. Those numbers may look similar, but while 5w indicates the same low-temperature fluidity, 30 and 40 represent different high-temperature viscosity grades. When mixed together, the oil film thickness becomes uneven, reducing engine protection at high RPMs and potentially causing accelerated component wear or increased noise. I once saw a neighbor do this, and the result was skyrocketing fuel consumption - the repair costs far exceeded the savings on oil. It's always best to use the same type of oil and double-check during oil changes rather than taking shortcuts by mixing. Especially for new cars with high precision components, extra caution in maintenance is crucial to prevent shortened engine lifespan. Maintaining consistency is key to keeping your vehicle running smoothly and reliably.
I think mixing 5w30 and 5w40 carries considerable risks. As a car owner, I pay special attention to vehicle safety. The main difference between these two oils lies in their high-temperature viscosity, and mixing them can easily lead to uneven lubrication or unstable oil pressure. Adding a small amount in an emergency for a short time might be harmless, but prolonged use can cause faster wear under heavy engine load and increase the risk of overheating. A friend of mine mixed them once, and his car's starting power weakened, costing him hundreds in repairs. I recommend developing good habits: check maintenance records, choose the recommended oil type, and don't risk big losses for small gains. If you find different oils in your car, replace them with a uniform type immediately—don't wait until problems arise to regret it. Proper maintenance ensures your car remains safe and reliable for a long time.
