All engines suck a small amount of oil out of the crankcase vent all the time. Esp when you have the most basic of basic of baffles in the valve cover like these things do. The spinning cam creates an oil "cloud". Even a low compression freshly rebuilt engine. Also a small amount of water condensation builds up a catch can will separate, leads to the oil lasting longer.
I'll say I've rebuilt countless engines over the years and since I started using catch cans 15 or so years ago the amount of grime and carbon deposits has been cut to near nothing. Oil tests (in my race engines) has proved the oil was cleaner at changes vs not running one. Enough to prove to me they have their merits. Why I run one on everything from my lawnmower to my daily drivers now. Everything 4 stroke that is.
Blow by is compression leaking past the rings and burnt gases pressurizing the crankcase blowing oil and exhaust out the vent. A whole different ballgame and even a catch can will not help there. If you only ride a few minutes and filling up a half pint catch can you know something is wrong.
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