The AC compressor pumps refrigerant in a repeating cycle through the AC system. As the refrigerant is pumped, compressor oil also travels with the refrigerant. The AC compressor is one of the few components in an AC system that has any moving parts to wear. In general, new compressors contain a correct charge of oil for your system, but re-manufactured compressors may only contain trace amounts of oil and need additional oil at time of installation. We only sell new compressors, so there is no need to add oil. Always replace the receiver/drier when replacing a compressor.