The word viscosity describes how easily an engine oil flows. Here we explain what descriptions like 0W-20 and 5W-40 actually mean.
The viscosity of an oil is measured by its resistance to flow. There are two numbers that define the viscosity of an oil. The first number ends with the letter 'W', which stands for Winter. This measurement is related to how an oil flows when it is cold, such as at engine start-up. The second number is defined by how an oil flows at normal engine operating temperatures.
The smaller the number, the better it will flow. So a 5W-30 will flow easier than a 10W-30 at start-up temperatures and a 10W-30 will flow easier than a 10W-40 at normal engine operating temperatures. This is important, as engine oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity oils flow easier to protect engine parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures.