Remove the two slotted screws in the lower unit and let the old L/U oil drain out. Refill with fresh lube making sure to use new plastic washers on the screws. If it is an electric shift use Type C gear case lube and for a manual shift use 85W90 gear lube.
Looking at your pictures on your other post regarding this it looks like you have the manual shift gearcase on a mid 1960s V4. If you are worried about a little milky oil, just refill the case and drain it a couple of times. wasteful of of oil but if it makes you feel better... That said, Water cooled gears and bearing are a good idea. Vacumn and pressure test the unit to find the leak. Quite often it is the seals under the water pump that let the water in, pushing ou the oil wherever it can
is the water pump the impeller? (i no, its a really dumb question), just wondering because i bought the seal kit for the gearcase from here and i was just wondering if the seals under the water pump that your talking about are located in the kit that i bought. Im also going to replace the impeller because i dont know when it was done last...