Originally Posted by jr_ss

Wrong. You have positive pressure in your crankcase, hence the factory catch cans... Pull your oil fill cap and tell me your hand gets sucked in and I'll call you a liar. Healthy engines have minimal to no air coming out, as engines age it increases over time.
Pulling a vacuum in your crankcase helps seal rings to the cyl walls and remove oily air from it. There are so many ways to argue this though. However YOU want to run it will be fine if YOU'RE happy.