What is the w in 5w30 oil

WHAT DO VISCOSITY NUMBERS MEAN?

The viscosity of an oil is measured by its resistance to flow. There are two numbers that define viscosity meaning. The first number precedes 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. A 5W-30 will flow easier than a 10W-30 at start-up temperatures and a 10W-30 will flow more easily than a 10W-40 at normal engine operating temperatures. Engine oil viscosity is important. Engine oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity oils give better protection to engine parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures.