A high powered modified engine tends to generate excessive blow-by oil and gasses. In these cases, an ordinary small catch can may quickly fill with oil and not be adequate. Like the Radium Engineering Catch Cans, the AOS-R (Air Oil Separator, Return) is responsible for keeping blow-by oil and other pollutants out of the air intake, intercooler, manifold, etc. The main difference is that an AOS-R system returns oil back into the pan above the oil level. This permits a service-free system.