The liquid should be filled the up and down limit line indicated the pump body side. But that the maximum level of liquid keeps below 20mm from tank top is better. If the liquid is over the maximum limit line, it happens the liquid dispersion. Reversely, the minimum liquid causes noise by air mixing or low performance of sucking particles from under tank. The discharging volume is sharply changeable by the viscosity. If the viscosity is high, the motor increases heat. So the viscosity of cutting oil keeps under 32 cSt. This pump is low head so that it can't discharge due to pipe resistance when the check valve is installed.