The short answer: The numbers represent the viscosity of the oil and the W stands for WINTER. … In a 5W-30 for example, the number before the W describes the viscosity of the oil at low temperatures. The lower the number, the thinner the oil and the better the oil’s cold temperature/ cold start performance
What do the numbers and letters mean on motor oil?
The letter “W” itself stands for Winter. As you may know, viscosity means the thickness of a fluid. In the case of motor oil, it is the thickness of the oil under certain temperature conditions. The number to the left of the W refers to the oil viscosity when the temperatures are low.