I have a 2001 302 with twin 5.0 Mercs. In the bilge, I found a blue plug (a T top with a screw bottom - picture attached). I see several of these plugs located on each engine (one in front and one on each side). At first I thought this plug is used to drain the water from the engine during winterization. But when I start the engines, I see no water running into the bilge and the engines do not overheat. Can you tell me what this plug is and where it may be located on the engine? Does it have anything to do with the outdrive? I ask because I seem to be loosing gear lube from the reservoir tank (from the full line to the fill line) each week when I am away from the boat and it is not in use.