A lubricator for a large slow running engine, the engine comprising a cylinder with a reciprocal piston inside and with a plurality of lubricant injectors distributed along a perimeter of the cylinder for injection of lubricant into the cylinder at various positions on the perimeter during injection phases; wherein the lubricator is configured for providing pressurized lubricant to the injectors in the injection phases; wherein the lubricator comprises a housing in which a hydraulic-driven actuator piston is arranged reciprocal along a stroke length, wherein the housing further contains a plurality of injection plungers and a corresponding plurality of dosing channels as well as non-return valves, one for each dosing channel, each injection plunger being slidingly arranged in one of the dosing channels, wherein the injection plungers are coupled to the actuator piston in order to be moved in common by the actuator piston over the stroke length and as a consequence pressurize lubricant in the dosing channels over a dosing channel distance for expelling the pressurized lubricant from each dosing channels through the corresponding non-return valve for injection of the lubricant into the engine cylinder; wherein the dosing channel distance defines the volume of lubricant expelled from the dosing channel through the non-return valve during the injection phase; wherein each non-return valve is provided in a corresponding valve-chamber in the housing and comprises a spherical valve-member pre-stressed against a valve-seat by the load of a spring; wherein the valve seat has a diameter of 3 to 10 mm; wherein the thickness of the ring-formed contact surface between the valve member and the valve seat is in the range of 0.01 mm to 0.5 mm, characterized in that the valve-seat has a circular symmetric shape wherein the circular symmetry varies by less than 0.2 mm when measured across the valve seat; wherein the surface roughness of the valve seat is less than 12.5 μm. wherein the lubricator comprises a housing in which a hydraulic-driven actuator piston is arranged reciprocal along a stroke length, wherein the housing further contains a plurality of injection plungers and a corresponding plurality of dosing channels as well as non-return valves, one for each dosing channel, each injection plunger being slidingly arranged in one of the dosing channels, wherein the injection plungers are coupled to the actuator piston in order to be moved in common by the actuator piston over the stroke length and as a consequence pressurize lubricant in the dosing channels over a dosing channel distance for expelling the pressurized lubricant from each dosing channels through the corresponding non-return valve for injection of the lubricant into the engine cylinder; wherein the dosing channel distance defines the volume of lubricant expelled from the dosing channel through the non-return valve during the injection phase; wherein each non-return valve is provided in a corresponding valve-chamber in the housing and comprises a spherical valve-member pre-stressed against a valve-seat by the load of a spring; wherein the valve seat has a diameter of 3 to 10 mm; wherein the thickness of the ring-formed contact surface between the valve member and the valve seat is in the range of 0.01 mm to 0.5 mm, characterized in that the valve-seat has a circular symmetric shape wherein the circular symmetry varies by less than 0.2 mm when measured across the valve seat; wherein the surface roughness of the valve seat is less than 12.5 μm.