What does 10W-40 actually mean? It's not a random code. It's the viscosity grade, and it tells you how the oil behaves at different temperatures.
The "W" stands for Winter
The number before the 'W' (10) indicates how the oil flows at cold temperatures. A lower number means the oil flows more easily when cold, making cold starts easier and reducing wear during warm-up.
The Second Number
The number after the 'W' (40) indicates the oil's thickness at operating temperature (100°C). A higher number means a thicker film, which is better for high-heat, high-load conditions.
Using an oil that is too thick when cold can starve the engine of oil at startup. Using an oil that is too thin when hot can lead to metal-to-metal contact. Always stick to the manufacturer's recommendation.