An oil catch can is used in turbo applications, or high-performance race applications where excessive blow-by (leakage past the piston rings) of air and fuel vapor occurs.
During engine operation, blow-by gases, as well as oil mist from the rotating components of the engine, pass through the PCV valve and are routed back into the intake for the engine to burn off. However, some of the oil mist and other products settle along the engine intake and over time form a "gunk."
The oil catch can collects the oil mist and condenses the fuel vapors while allowing "cleaner" gases to be passed back into the intake. Typically the blow-by gasses are passed through a wire mesh, which give the vapor droplets something to adhere to. the catch can catches all of this.
Used to run one until very recently, still was getting some oil into the plenum area. Ended up just yanking everything out and capping off the PCV. So far so good, a little whiff of oil vapor once in a while but not unbearable. I have two valve cover breathers.
Crankcase blowby gases have lots of stuff in them, including lots of water from the combustion process. A catch can provides a nice cool place for that water to condense and make some slimey oily stuff which you will have to clean out periodically. Don't let it freeze in cold weather cus the ice could block the ventialltion system and make the crankcase pressure build up.
I prefer to run open breathers. One on each valve cover. PCV plugged up and not used.
I plugged the breathers off on the valve covers using grommets that had no holes in them. The breather were there just for appearnace (to fill the holes in the valve cover). I enlarged the hole in the intake manifold which retained the PCV valve. Then put a -12 fitting with an air restricting plate inside the intake manifold. This grabbed all the crank case pressure right from the lifeter valley area before ever traveld up the heads to the breathers. Routed this hose to catch can with a breather. Damn good ring seal is also needed, but it is impossible not to vomit oil all over the place when crank case pressure from a 7,000 RPM HIGH PSI BOOSTED V6 wants out.
For a catch can to be effective it must have some type of median/ filter in it to "catch" the oil and moisture droplets in their vapor form. There are a ton of catch cans on the market that do not have anything in them, just a hose going in and a hose coming out which will catch nothing. The couple I have that work have a sort of coarse Brillo type stuff in the top that the vapors travel thru and get "caught" on, can fills up in about a thousand miles, quicker when it's cold out from the condensation.
I don't remember what company made mine I think they sold off their patent of their design, it was all billet aluminum and another company now sells the design for almost twice as much now, was about $100 back then. For non-turbo applications a large air compressor moisture separator works very well.