A lower unit has about an inch of air space above the level screw to allow fo expansion when running and the oil gets hot, many drive shafts have a spiral cut in them so as they turn it slings oil up to the top driveshaft bearing, thus forcing the air in the top down into the oil airating the oil. The only true test for water intrusion is , after running the motor on the lake , tilt it down and let it sit for a couple of days, any water will go to the bottom and be visable when you crack the bottom screw loose (like an oil spill water is heavier than oil) and the time allows all the air to work it's way out and back to the top. Johnson / Evinrude motors should hold 15 psi on a presure gauge for at least 20-30 mins and Merc/Mariners won't hold over 7-8 psi before you blow the shift rod bushing seal out