The number in front of the “W” signifies the viscosity of the oil at cold/start up temperature (tested at -10 to -35°C) depending on the grade. Please refer to the SAE J300 – Engine Oils Chart opposite). The lower the first number, the faster the oil flows (pumpability) when the engine is cold.
The second number represents the oil’s thickness at operating temperature (100°C). All oils thin out as they get hotter. So, the higher the second number, the less the oil will thin out as it heats up, compared to an oil with a lower second number.
Normally, multigrade viscosities are measured on a curve.