So, what do those numbers on the bottle actually mean?
Viscosity is defined as a fluids resistance to flow, the unit of measurements that we are concerned with in classifying engine oils are cSt (centistokes) and cP (centipoise). The numbers you see on the bottle are from the SAE J 300 grading system and indi
cates that the oil meets specific viscosity requirements; Cranking viscosity test indicates that the viscosity is low enough at low temperature to allow the engine to crank fast enough to start. Pumping viscosity indicates the oil will have a low enough viscosity at low temperature to allow the oil pump to move it through the engine. If an oil is a multigrade, two additional requirements must be met: the oil must meet the kinematic viscosity requirements at 100°C and a High Temperature High Shear Test run at 150°C.
Looking at the chart above if we were to select an SAE 5W30 engine oil we can see that this oil would be required to have a Cranking Viscosity, 6600cP max at -30°C, a Pumping Viscosity of 60,000 cP at -35°C, a Kinematic Viscosity of between 9.3 cSt and 12.5 cSt at 100°C and a High Shear Viscosity of 2.9 cP min at 150°C.