Viscosity (a fluid's resistance to flow) is rated at 0° F (represented by the number preceding the "W" [for Winter]) and at 212° F (represented by the second number in the viscosity designation). So 10W-30 oil has less viscosity when cold and hot than does 20W-50. Motor oil thins as it heats and thickens as it cools.
Consequently, what is meant by viscosity of oil?
Viscosity, by definition, is an oil's resistance to flow and shear. It is the single most critical physical property of the oil as it affects both the wear rate and the fuel efficiency. Water is a low viscosity fluid; syrup is a high viscosity fluid.
What is viscosity grade?
Oils can be separated into multigrade oils and monograde oils. Multigrade oils must fulfill two viscosity specifications, their viscosity grade consists of two numbers, e.g. 10W-40: 10W refers to the low-temperature viscosity ("Winter"), 40 refers to the high-temperature viscosity ("Summer").
How do you measure the viscosity of oil?
Stating an oil's viscosity is meaningless unless the temperature at which the viscosity was measured is defined. Typically, the viscosity is reported at one of two temperatures, either 40°C (100°F) or 100°C (212°F).