Boat engines in March
Second thing is drain the water from lower unit when done. Yes they are self draining, what I do, is lower the motor to the running position on the trailer while I am tidying things up/ Then pull kill switch, turn motor over a couple times to pump any left over water out. Trim motor up and you are good to go.
With the top plug in place, unscrew the bottom plug (don't remove it, just loosen it) and see if any water comes out.
Oil floats on water, so if any water comes out you know that you have a leaky seal. If just oil comes out you are in good shape.
Also, if water does come out, drain your lower unit completely, and refill with oil. Then take you motor into a marina for a pressure test to see where the leak is.
Also make sure your seals in gear case are good,with no water in lube.