The number on the package illustrates the viscosity grade of the product you are purchasing.\rXXW-YY\r5W-40\r\r5 indicates the viscosity at low temperatures or winter temperatures. Most of the kinematic viscosity measurements are @ 20°C. \rW Stands up for operating in Winter conditions. \r40 illustrates the viscosity at high temperatures. Most of the kinematic viscosity measurements are @ 100°C.\rNote: The bigger the difference between XX and YY, the more stable the viscosity of the oil is.