The oil-free screw compressor element
No oil means that there is absolutely no oil for sealing the rotors and for cooling the compressed atmosphere, components and rotors. ... The component is certainly Oil-free Screw Air Compressor cooled by cooling water that flows 
through unique pockets in the component casing.
In a lubricated screw compressor, the lubricant will come in direct in touch with the compressor inlet air and compressed air. ... The oil contains anti-oxidants to try to reduce the break down of the lubrication properties because of internal conditions.