5w40 and 10w40 are two different grades of motor oil. The “W” in the grade stands for “winter,” and the number before the W indicates the oil’s viscosity at low temperatures. A lower number means that the oil will flow more easily at low temperatures. Consequently, oil with a lower number, like 5w40, will perform better in cold climates and during cold startups than oil with a higher number, such as 10w40.
The number after the W indicates the oil’s viscosity at high temperatures. A higher number means that the oil will be thicker at high temperatures. Thicker oil can provide better protection against wear and tear, but it can also reduce fuel economy. Oil with a higher number, like 10w40, is better suited to hot climates and high-performance driving.