Oil grades explained: what 5W-30 actually means
Two numbers and a letter decide whether your engine is protected on a January morning or starved at motorway temperature.
The first number is about cold starts
The figure before the W describes how the oil behaves when the engine is cold. Lower is thinner, and thinner oil reaches the top of the engine faster on start-up, which is when the majority of engine wear happens. A 0W oil is not a luxury on a modern engine, it is what the tolerances were designed around.
The second number is about operating temperature
The figure after the dash describes thickness once the oil is at around 100°C. This is where the oil has to hold a film between the bearing and the crank under load. Too thin and the film breaks down, too thick and the pump struggles to circulate it through narrow galleries.
Thicker is not safer
The most common mistake is treating a thicker oil as extra insurance. Variable valve timing units, hydraulic tensioners and turbo bearings all depend on oil passing through fine passages at a predictable rate. A thicker grade slows that down and can cause the exact wear it was meant to prevent.
When a substitute is acceptable
Some manufacturers publish a fallback grade for when the specified oil is unavailable, usually one step thicker and only until the next service. Toyota permit 0W-20 in place of 0W-16, for example. Treat these as a way of getting to a service, not as an alternative specification.