What's the difference? I thought separators and catch cans were different names for the same thing. The oil/air mist first goes through the separator portion, and then drips down to the can at the bottom. Every few months you can remove the can and drain the oil (some have valves that let you drain without removing).I used one years ago. You don't want a separator you want a catch can. It grabs probably 3 times as much oil that a separator does.