Oil-lubricated compressors are "bathed" in oil to reduce the wear and tear caused by the constant pumping of internal pistons. As a result, the pump resists overheating, runs longer, and performs better. However, these compressors require maintenance i.e you must check the oil level on a regular basis to ensure that the proper amount  is delivered to the compressor piston. Otherwise, the motor will overheat, become damaged and cease to work. It also very important to use the correct oil i.e synthetic , non-petroleum oils. Mineral or petroleum based oils will corrode the "REED" valve of the compressor which is responsible for sucking in the ambient (air we breathe) air into the compressor to be "compressed". This will result in costly repair. With an oil-less compressor, there is NO maintenance required. These compressor motors are permanently sealed and lubricated. In essence they "self-lubricate".