Gear oil or engine oil?
If it's gear oil and it's a VP SX or DPS, are you refilling it from the top or the bottom?; they should be filled from the bottom, to stop airlocks. If you fill it from the top and stick the dipstick in, it might look full or even over full to begin with, but the level drops quite slowly over time as the air rises to the top. If that's not the cause, yes, it could drip past the propshaft seal but I'd have expected some oil to witness after it was standing a while in your driveway, especially if there was some air pressure in there from it getting hot.
If it's a Mercruiseer, I have no idea or experience.
If it's engine oil and crankcase pressure you're talking about, first thing to check is for a blocked crankcase ventilation valve.
Graham
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