When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I had a vented catch can on my 914-6 and never had anything in the can. Some people I know that ran that same motor (Carrera 3.2) on the track would get oil in theirs, but I never tracked my car.
Search GT4 forum for posts by d00d. He has an extensive thread about a vent to atmosphere, and drip to ground, of any oil that might come out. Also some info in there about some high quality air-oil separators that can be installed in the crankcase vent line.