10w-40 oil is a multi-grade oil, meaning that it has a viscosity range that covers both cold and hot operating conditions. The "w" in the designation stands for "winter," and it refers to the oil's viscosity at cold temperatures. The first number, in this case 10, indicates the oil's viscosity at 0 degrees Celsius (32 degrees Fahrenheit). The second number, 40, indicates the oil's viscosity at 100 degrees Celsius (212 degrees Fahrenheit).
Two-stroke dirt bikes typically require oil that has a lower viscosity at cold temperatures to ensure proper flow through the engine. Using an oil with a higher viscosity, such as 10w-40, can make it more difficult for the oil to flow properly at cold temperatures, which can lead to engine damage. Additionally, 10w-40 oil may not provide adequate protection for the engine's bearings and other components at higher operating temperatures.
It is important to use the type of oil that is recommended by the manufacturer of your dirt bike. This information can usually be found in the owner's manual or on the manufacturer's website. Using the wrong type of oil can void the warranty on your dirt bike and can also cause significant engine damage.
