Anything that penetrates the block and leads to the crankcase can eventually leak and let air in or oil out, if oil is coming out air can definitely get in. The crankcase on modern cars is under negative (vacuum) pressure to prevent the gases from 1. building up to a high pressure and forcing oil out the seals 2. burn the gases from combustion and any vapors in the crankcase so they dont escape to the environment and all this is accomplished through a PCV (CDR on a diesel) valve that only opens when there is correct vacuum applied to the valve from the manifold, the amount of air that can normally be scavenged from a crankcase is usually small so on some cars when you remove the cap the engine will idle noticeably higher due to the extra air in the system now, some may even trip the MIL light due to a lean condition due to the extra unmetered air flowing into the engine if it is fuel injected. All that being said if someone knows otherwise that is what I know about that system but if something is wrong I hope someone will say so Anyways if you have a leak at your oil cap you will get unfiltered dirty air into your crankcase that WILL slowly contaminate your oil and lead to premature engine wear.Well, you have the notorious rubber gromet in the valve cover, many times a new gromet makes the engine idle better. I guess that would be the same type of vacum leak, as a leak at the dipstick?
Sorry, have no experience with the turboed cars, mine is unfortunately a gutless turdTrionic 5 does not have a mass air flow sensor, thus you aren't letting in any "unmetered air". As long as the PCV valve on the vacuum line between the PCV nipple and the throttle body is good, you'll be fine.
The worst that's going to happen is oil vapor and water vapor may condense on things near the cap and make a mess.