With the increase and decrease in temperature, the oil inside the engine will get thicker. And the numbers state the thickness, weight, or viscosity of the oil in that different temperatures. Let us take 0W40 as an example. First of all, the first numbers, which is 0W, state the viscosity of the oil in extremely low temperatures. The 40 refers to the thickness of the oil when the engine is combusting, which means the temperature is rising.