ISO and SAE stand for the International Organization for Standardization and the Society of Automotive Engineers, respectively. ISO and SAE have provided two distinct grading systems governing the viscosity measurements of motor oil.
Viscosity is the measurement of how much a fluid resists different kinds of stress. The more viscous a fluid is, the more resistant it is to stress and movement. For example, tomato juice is more viscous than water.
The ISO system uses a viscosity measuring approach that is designed according to the number of centistokes the fluid presents at 40 degrees Celsius. A centistoke is a measure of a fluid's kinematic viscosity. For example, ISO grade 32 will be measured at 32 centistokes at 40 degrees Celsius. The SAE system uses an entirely different approach centred on how long it takes for a certain amount of oil to flow through a certain space at certain temperatures. The longer it takes to flow, the higher the viscosity.
Most ISO grades have equivalent SAE grades. ISO 32 = SAE 10W, ISO 46 = SAE 20, ISO 68 = SAE 20, ISO 100 = SAE 30, ISO 150 = SAE 40 and ISO 220 = SAE = 50.