I cobbled up a separator for a worn out Ford Escort to reduce the amount of oil that ended up in the air cleaner. Worked OK. Then one really cold evening, I get a call from the wife "The good news is I made it to work. The bad news is that the engine is making loud knocking noises and it won't go over 30 mph." I drive over to see the car sitting in a puddle of oil. I assumed it tossed a rod. Next day (when it's light out) I go back, look around, don't see any holes in the block. So, I add a few quarts of oil, and fire it up - what do I have to lose? Clatters for a couple seconds and settles down and runs. Sheeeeeit. I can get this thing home. Had to stop and add a couple quarts on the way, but I made it.
Turns out that (apparently) the oil separator must have frozen because the camshaft seal had been blown out of the head. Removed the separator, Put in a new seal, drove it for several more years.
Last time I tried to put a homemade separator on a breather line.