Radium Air Oil Separator w/RHD Bracket - Subaru WRX/STI 01-07A 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 (Air Oil Separator) kit is responsible for keeping blow by oil and other pollutants out of the air intake and intercooler (if equipped) systems. The main difference is that an AOS system returns collected oil back into the oil pan so