Circulating completion, workover and drilling fluids used in hydrocarbon recovery are filtered after monitoring for viscosity, which frequently causes plugging of filters. A viscometer generates a signal representative of viscosity in the fluid; the signal is used by a programmable controller to divert viscous fluid from the filter, or to take other action to prevent damage to the filter. The viscometer can be used in various positions in the system. Fluids deemed too viscous for the filter can be sent to a viscosity-reducing device, which may be a heating, shear-thinning, or cavitation device, to reduce its viscosity, enabling the fluid to pass through a filter without fouling. After filtering and a return to a lower temperature, the fluid may be treated if necessary to become viscous again for a useful purpose.