The oil-free screw compressor element
No oil implies that 
there is Oil-free Screw Air Compressor absolutely no essential oil for sealing the rotors and for cooling the compressed air, elements and rotors. … The element is certainly cooled by cooling water that flows through special pockets in the element casing.
In a lubricated screw compressor, the lubricant comes in direct in touch with the compressor inlet air and compressed air. … The essential oil contains anti-oxidants to try to reduce the breakdown of the lubrication properties due to internal conditions.