High-powered modified Toyota 4U-GSE and Subaru FA engines tend 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 Dual Catch Can Kit, the AOS-R (Air Oil Separator-Return) kit is responsible for keeping blow-by oil and other pollutants out of the intake pipe, manifold, turbocharger, and intercooler during all engine loads. The main difference is that an AOS-R system returns what is collected back into the oil pan (as shown below) so there is no servicing required. The built-in heater keeps water from condensing in the AOS-R. And because water is more dense than oil, there is an integrated water trap found around the bottom perimeter of the can.