The oil doesn't go past the piston ring. The air you suck in goes from the throttle body, to the intake manifold, to the top of the block where it mixes with fuel and ignites. You won't see any indication of damage in the oil (unless you blew out a piston/ring which you would def know).
No worries I'm sure you are fine... 300mi isn't really that much to have a small air leak.
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