The letter “W” stands for winter. The first number in an oil classification refers to cold weather viscosity. The lower the number, the less viscous (or thick) your oil will be at low temperatures. A 5W- motor oil, for example, will flow better at low temperatures than say, a 15W- motor oil. The second number in the classification, the number after the “W” refers to hot weather viscosity, meaning how the oil flows at hot outdoor temperatures. The higher the second number is, the thicker the oil will be at a specified outside temperature.