Yesterday we began describing how to change the oil on the lower unit of a Mercury outboard. Picking up from where we left off, remove the upper vent plug and set it aside. Oil should now be flowing out of the lower unit and into the drain pan. This can take one to two hours for the unit to fully drain.
Take your supply of Mercury engine oil and dispense it into the lower unit. Continue adding oil until it flows from the top vent plug opening. Once that happens, replace the top vent plug and the drain plug. Let the oil settle overnight, check the levels in the morning, and add more oil if need be.