The reason the oil looked new when you bought it is the previous owner
"probably" changed the oil and never ran it in water after that.
= hence no water in the oil when you bought it.
with many thanks to Pappy, I was able to overcome
my intimidations of breaking open the gear case . . .
To replace the prop shaft seal is pretty straight forward.
To dig down and replace the shift linkage seal is a bit more complicated.
Order the complete Gear Case Seal kit from e-bay or Marine Engine
https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1985&hp=15&model=E15ECOB&manufacturer=Evinrude§ion=Gearcase
and using your parts book, take it all apart, taking photos as you go.
clean as you go - take notes as you go.
don't muck up the head on the pivot pin or loose the detent spring or the two little balls.
work on a clean table (the kitchen table is perfect for this).
the good old YouTube has several good videos on how to pressure test to check for leaks
and to replace the seals with a seal kit.
oh, it is also highly recommended to replace the impeller while you have it open.
.