Why is viscosity an important property of matter?

Viscosity, according to Webster's Dictionary, is the property of fluids and semi-fluids that defines its internal resistance to flow and shear ... Also, as hydraulic fluid ages or experiences shear stress, oil molecules can break down and that lowers the viscosity.

It governs the sealing effect of oils and the rate of oil consumption as well as determines the ease with which machines may be started or operated under varying temperature conditions, particularly in cold climates. viscosity is a measure of an oil's resistance to flow.