Can 30w and 40w motor oils of the same brand be mixed?

[DelTaylor]

They cannot be used together. Mixing them will increase engine cylinder wear, lead to carbon buildup, and shorten the engine's lifespan. The two oils are significantly different, and using the wrong oil can negatively affect your vehicle. Differences between the two oils: The main difference between 5W30 and 5W40 lies in the oil viscosity. Both being 5W, they have the same low-temperature performance. The numbers 30 and 40 represent the oil viscosity values—the higher the number, the greater the viscosity. However, viscosity alone cannot determine oil quality; the most important factor is choosing the right oil for your vehicle. Effects of using the wrong oil: It may lead to higher fuel consumption, heavier throttle response, insufficient power, engine shaking, increased noise, or even failure to start. More critically, it can damage engine components and reduce the engine's service life.