Viscosity of Lubricants

viscosity:  vis·cos·i·ty  [vi-skos-i-tee]

  • the state or quality of being viscous.
  • the property of a fluid that resists the force tending to cause the fluid to flow


Motor oil viscosity is a measure of how resistant the oil is to flow.  Higher viscosity oils are thicker and flow more slowly through a viscometer than lower viscosity oils. Some motor oils are tested for viscosity when they are cold. These oils’ viscosity is marked with a W (i.e. 10W-30). Oils that are low viscosity when cold are desirable because the car’s engine needs immediate access to oil while still cold. Low viscosity oils will lubricate the engine when it is first started.

Additives can increase oil viscosity so that it does not thin out as it heats up. Additives are added to thinner oils to raise the viscosity at lower temperatures. Then, as the oil heats, it thins to the proper viscosity instead of thinning so much that it hurts the engine. However, as the oil thins, the additive breaks down; eventually the oil will be thinner than usual at high temperatures instead of thicker.