Viscosity

Viscosity is a basic characteristic of engine oil. The higher an oil's viscosity, the thicker the oil film formed on the metal surface. Likewise, the lower the viscosity, the thinner the oil film.
Also, because viscosity changes depending on the temperature, viscosity will be low when the temperature is high, and high when the temperature is low.
This means that it is important for engine oil to have the proper viscosity and not suffer significant fluctuations in viscosity due to temperature changes. In other words, it is best for engine oil to have a high viscosity index.
In recent years, multi-grade engine oils are sold that suffer minimal fluctuations in viscosity due to temperature changes. These multi-grade oils can be used across a wide range of low and high temperatures and in all seasons. (10W-30, 20W-20, etc.)
*Oils that suffer minimal fluctuations in viscosity due to temperature changes are those with a high viscosity index.