Oil Separator for Crank Case Vent Line
Aircraft engines don't use tightly fittet pistons as they are old air-cooled designs with large temperature spread. Therefore there is a lot of blow-by past the pistons ending in the crankcase. The stock solution is to use a 1 inch hose and just dump those fumes out the bottom of the engine compartment leading to oil and dirt accumulating on the bottom of the fuselage.
There are designs that separate the oil and return the oil into the oil sump and only dump the cleaned fumes out the bottom. Drawback is that not just oil condensates in the separator but also water and therefore water is returned back into the oil pan. This happens when the oil separator is cold in the beginning and potentially during the entire flight if installed in vented/colder area of the engine compartment. See this video about how much water accumulated in a catch can.
To further clean the remaining fumes coming out of the separator, some opt to feed them into the exhaust pipe. Because the exhaust is very hot, burnt residues build up at the adapter which needs to be cleaned out at every oil change. To rule out clogging, it is necessary to also install a bypass valve that opens should the exhaust feed close up.
There are other designs that just use a reservoir (with overflow hole) to hold the separated oil / water for manual dump during oil change.
See this Kitplane article by Dan Horten
See this VAF Thread
See also this VAF Thread
See also this VAF Thread (with pictures of clogged exhaust adapter)
See also this FB Thread