The oil-free screw compressor element
No oil implies that 
there is no oil for sealing the rotors and for cooling the compressed air flow, components and rotors. … The Oil-free Screw Air Compressor element is cooled by cooling water that flows through particular pockets in the element casing.
In a lubricated screw compressor, the lubricant comes in direct in contact with the compressor inlet air and compressed air. … The oil contains anti-oxidants to attempt to reduce the break down of the lubrication properties because of internal conditions.