What do the viscosity numbers mean on motorcycle oil labels?

The viscosity of motorcycle oil is measured by its resistance to flow. There are two numbers that define the viscosity of motorcycle oil. The first number ends with the letter ‘W’, which stands for Winter. This measurement is related to how motorcycle oil flows when it is cold, such as at engine start-up. The second number is defined by how motorcycle oil flows when at higher temperatures normal engine operating temperatures. 

The smaller the number, the better it will flow. So a 10W-30 will flow easier than a 20W-30 at start-up temperatures and a 10W-30 will flow easier than a 10W-40 at normal motorcycle engine operating temperatures. This is important, as motorcycle engine oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity motorcycle oils flow easier to protect engine parts at cold temperatures. Thick, high viscosity motorcycle oils are typically better at maintaining film strength to protect motorcycles engines at high temperatures.