**s****hear ****s****tress **- frictional force overcome in sliding one "layer" of fluid along another, as in any fluid flow. The shear stress of a *petroleum *oil or other *Newtonian **fluid *at a given temperature varies directly with *shear rate *(velocity). The ratio between shear stress and shear rate is constant; this ratio is termed *viscosity. *The higher the viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear. In a *non Newtonian **fluid *- such as a *grease *or a *polyme**r**-co**nt**aining *oil (e.g., *multi-g**r**ade oil) *- shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an *apparent **v**i**scosity**, *a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined. See *Brookfield **v**i**scosity**.*