The rotary-screw compressor is available as oil-injected and oil-free versions. The basic principle is the same (the rotors “push” the air to one side), but they are quite different machines.
Oil-inject models are by far the most common oil-injected screw-type compressors. When you need a lot of air in your workshop or factory, this type of compressor is usually the best choice!
Oil-free models are used for specific special applications. I’ve mostly seen them on big factories like oil/gas or chemical refineries, big food factories, or other places where the compressed air must be 100 percent oil free (otherwise it could contaminate the food, product, or chemical process).