As long as I am not getting water in the lower unit I leave the oil. I usually fish all winter. As I come out of the water and start to make the boat ready for the trip home. I tilt the motor all the way down and let the water run out of it. Things I have experienced while fishing in the winter. Cold enough for the boat ramp to ice over. If you are the first guy in it isn't a problem. As you pull the trailer out the water can freeze on the ramp. Pretty scary feeling as your truck slides down the ramp slowly. It did stop when it got past the ice and the back tires got to the water. After that experience I keep ice melter in the truck for such occasions. So cold out the motor would not pee. Apparently the water hit the cold metal inside the motor and froze on contact. I believe it was 12 degrees that day.