What do the letters and numbers on a bottle of engine oil actually mean?
The numbers on the side of any bottle of engine oil refer to the SAE viscosity rating at different temperatures.
For example for a 5W-30 engine oil, 5 refers to the SAE viscosity rating when the engine is cold and the ‘W’ stands for Winter.
Finally the second part, 30 refers to the SAE viscosity rating when the engine is at optimum operating temperature.
As the oil in your engine heats up it becomes thinner, multi grade engine oils thin less when the car engine is at its operating temperature. This allows the engine to maintain a constant oil pressure.