What does 0W-20 mean on engine oil?

It is a viscosity rating — a description of how thick the oil is, at two different temperatures. Viscosity is the single most important property of engine oil, because oil that is too thick cannot circulate quickly when cold and oil that is too thin cannot maintain a protective film when hot.

The first number with the W describes cold behaviour. W stands for winter, not weight. A 0W oil stays fluid at a lower temperature than a 5W, which stays fluid lower than a 10W. This matters far more than most drivers assume: the majority of engine wear happens in the first seconds after a cold start, before oil has reached the top of the engine. A thinner cold rating gets there faster.

The second number describes viscosity at operating temperature, around 100°C. A 20 is thinner at temperature than a 40. Modern engines with tight tolerances and variable valve timing are designed around thin oils, partly for fuel economy and partly because their hydraulic systems depend on the specified viscosity to work correctly.

Multigrade oils behave like a thin oil when cold and a thicker one when hot, which is what the two numbers together describe.

The important practical point is that the grade is not a preference. Use what the manufacturer specifies. Putting a thicker oil in an engine designed for 0W-20 can starve components at cold start and upset variable valve timing; putting a thinner oil in an older engine designed for 10W-40 can leave inadequate film strength when hot.

Specifications such as API SP or ACEA C5 printed alongside the grade matter too — they describe additive chemistry, not thickness, and some engines require them.