Viscosity is the most important parameter for the engine lubrication system. Viscosity is the resistance of liquid during its flow. The lower viscosity, the smaller oil resistance, and thus smaller loss in engine’s power. Whereas the higher viscosity, the better protection of engine against wear and tear.
An oil having a too low viscosity is less resistant to breaking the oil film at high temperatures, whereas Oil having a too high viscosity at low temperature may not reach lubricated elements or may reach them too late, and its oil film is easily broken at high rotation speed. The oil viscosity is determined according to SAE classification.
See also: Oil viscosity – how to read labels?