What do those numbers mean on oil?

We see numbers like 5W-40 printed on every bottle of motor oil, but what do they actually mean? They describe how the oil flows at different temperatures. The “W” stands for winter, not weight. The number before it indicates how easily the oil flows during a cold start. The lower that number, the better it performs when the engine and weather are cold.

The second number describes the oil’s viscosity once the engine reaches operating temperature. In 5W-40, the oil behaves like a 5-grade oil during a cold start and maintains the protection of a 40-grade oil when hot. That matters because oil must flow quickly when you first start the engine while remaining thick enough to protect hot, fast-moving components after you have been driving.

This does not automatically mean that 5W-40 is “better” than 5W-30 or 0W-20. Modern engines are designed around very specific oil viscosity and performance requirements. Using oil that is too thick can reduce circulation and efficiency, while oil that is too thin may not provide the protection the engine was designed to receive. Climate, engine design, mileage, turbocharging and driving conditions can all play a role.

The safest answer is still the least exciting one: use the viscosity and specification listed in your owner’s manual. The numbers on the bottle are not a ranking system. They are instructions for how the oil behaves, and your engine’s engineers already did the expensive testing so you do not have to conduct your own experiment on the freeway.

Oil viscosity can directly affect engine performance because the oil must move quickly through small passages while maintaining a protective film between metal surfaces. Oil that is thicker than recommended can create additional internal resistance, reduce fuel economy, slow lubrication during cold starts, and interfere with systems that depend on precise oil pressure, such as variable valve timing and turbocharger controls. Oil that is too thin may flow easily but fail to maintain sufficient protection under high temperatures and heavy loads, potentially increasing wear, oil consumption and engine noise. The correct viscosity helps the engine start smoothly, respond properly, maintain consistent oil pressure, and deliver the balance of power, efficiency, and protection its designers intended.