water in gear case johnson 1992 6hp

A buddy of mine gave me a 1992 johnson 6hp sailmaster outboard to use on my 12 ft boat the motor was stored for a couple of years before i got it but we tested it and it runs like a champ. So we took it out 4 different times with no problems the last time out we were trolling along and the engine started getting hot so I shut it down and brought it back home after pulling the gear case apart and finding a broken impeller. After pulling the drive shaft out of the gear case I noticed that the oil in the gear case was milky looking.I'm hoping this was caused by the impeller not working, I am not a mechanic but i was thinking that when I replace the impeller and put this back together I should be able to drain gear case and refill it with motor oil (just to flush it out a bit) and run it in the barrel for a sec then drain it If its still milky then I am thinking it's prob my seal in the gear case unit that the drive shaft runs through.....let me know if I am on the right track THanks