Re: 15HP Evinrude leaking gear oil.
If you determine it is gear oil, then it is wise to replace the prop shaft and drive shaft seals, plus the shift rod o ring.
Here is how to do it without taking the gear case apart.
The prop shaft and drive shaft each have two seals back to back, they are all identical however. See below for how I just tackled this chore on my 1975 9.9/15:
Figure out how thick each seal is, as usually the prop and drive shaft seals are two back to back. Select a drill bit that will be the right pilot size for a 1.5 inch long deck screw. Mark the drill bit with tape to indicate how far you can drill into the edge of the seal and not go so far as to hit the aluminum casting below the seal. Dip the bit in grease to catch any chips it will be making. Now carefully drill into the outer metal part of the seal until you are at the taped mark on the bit. Next, screw in the deck screw into the hole you just drilled and using a small flat board and a claw hammer, pull on the head of the deck screw and remove the seals. If you do it carefully, you will go through both seals with the screw and they will come out together, easy as pie. Of course, you destroy the seals, but if they were good they would not be needing removal anyway!
For a shift rod seal, unthread the shift rod, and remove it. Get a new shift rod bushing, washer, and o ring seal. Tap the shift rod bushing with a tap dipped in grease, then thread a long piece of all-thread into the bushing. Now you can take a bar of steel with a hole in it for the all-thread and place it across the gear case top with the all-thread going through the hole. Run a washer and nut down the all-thread until it hits the steel bar and begin tightening the nut to pull the shift rod bushing out. Put grease on the new oring and washer and insert them into the cavity where the old o ring and washer were, and drive home the new shift shaft bushing on top of them, Replace shift rod and thread it in place.
