Originally Posted by HangFire
The purpose of the catch-can is to prevent motor oil from becoming deposits on the intake valves. Along the way it also catches water vapor, and breakdown products of oil. I suppose, for a zero-maintenance system, that returning all that to the sump would allow it to be removed in the next oil change. All of those things are already present in used motor oil anyway.
Since this is the stuff that "boiled out" of the oil, it just didn't want to be there in the first place. Returning it to the sump gives it another opportunity to boil out in a never-ending cycle. If we had a problem of losing an entire quart of oil to "evaporation" and the catch can held most of it, I'd say put it back... otherwise, no thanks. (And I wouldn't want something capable of evaporation lubricating my engine anyway...)