An apparatus for the control of the viscosity of fuel oil supplied to burners, which comprises, in series, pump means for supplying the fuel oil to the burner system under a selected pressure, means for heating the fuel oil output of the pump means, strainer means and control valve means and orifice means of selected length and diameter. A differential pressure cell monitors the pressure drop across the orifice due to the flow of fuel oil under a selected pressure set by the control valve. Any variations in differential pressure will be due to corresponding inverse variations of viscosity. The variations in the output of the differential pressure cell are communicated to a valve means which controls the amount of steam passing into the heating means, thus controlling the temperature of the fuel oil, and therefore its viscosity, to a proper value.