What does the letter W mean?

The letter "W" means winter. The first number in the classification indicates that the oil can be used in cold weather. The lower the number, the less viscous the oil will be at low temperatures. For example, 5W-30 engine oil will be more volatile at lower temperatures than 15W-40 engine oil. The higher the number following the "w" refers to the viscosity in hot weather or to, and the higher the number, the thicker the oil at the specified temperature.