There are special marine lower unit oils that are said to be formulated a bit differently for the challenges of marine life….and supposedly better able to handle small amounts of water intrusion.
Initially I would just use a good quality 90 oil and monitor it closely for the first few runs to see if it turns milky (water intrusion). If no water intrusion….then you might opt for the better stuff…..
Those gearcases frequently leak (the shift shaft seal is the most common culprit….but there are seals on the driveshaft and prop shaft as well.) All available if you decide to go thru the gearcase.
