Disclosed herein are compositions of a manganese sulfide (MnS) compound useful as additives for making a sintered product. Also disclosed herein is a method of making the composistions in which molybdenum (Mo) or Fe—Mo is added to the MnS compound to improve machinability and to obtain a more stable MnS compound, thereby reducing any change in weight and size in a sintering process. The compositions can suppress erosion of parts in a sintering furnace during a sintering process, prevent sooting on a surface of the sintered product from occurring, and enhance resistance to moisture in the air to keep the sintered product in the air for a long time.