Winterizing is an important part of boat ownership, one that can be critical to the long-term performance of engines, lower units and other components. Gear oil is intended to emulsify water and leaving used oil in the lower unit will lead to separation, and eventually rust and corrosion. In extreme cases, if there is enough water in the lower unit it could freeze and crack the housing. Gear oil holds soot and contaminants in suspension and allowing used gear oil to sit all winter could cause those contaminants to drop out, resulting in sludge buildup.
Marine engines often run cooler and are subject to moisture buildup in the crankcase. Changing engine oil is critical to preventing rust and corrosion during winter storage. Two-stroke engines should be fogged to prevent corrosion during storage.
Be sure to grease steering linkage and it’s also a good idea to grease the prop shaft to prevent corrosion problems between the prop and the drive shaft. Treat any fuel remaining in the fuel tank to prevent moisture problems and fuel oxidation.