Abstract: Apparatus and method for measuring the viscosity of a fluid by drawing a sample of the fluid through a first flow restrictor (24) and forcing the sample through a second flow restrictor (28) having flow restriction characteristics which are identical to the flow restriction characteristics of the first flow restrictor. The difference in pressures of the sample at the exit of the first flow restrictor and at the entrance to the second flow restrictor provides a measure of the viscosity of the fluid and may be used to vary the temperature of the fluid to control its viscosity.