Question: My dock buddy and I (he owns a 56-foot Californian with Caterpillar 3208s, and I own a 50-foot Hatteras with Detroit Diesel 8V-71s) use our mains only about 45 or 50 hours a year on a Midwestern lake. Each boat takes 12 gallons of oil, a couple of filter elements, and a fair amount of work to change the oil in the fall. Guidance from the manufacturers specifies a 150-hour oil-change interval. Should we do end-of-season oil changes or can we stretch the old oil into the next season? And what about changing the oil but not the filters?

— Tom Hammelman St. Louis, Missouri

Professor Diesel: Most manufacturers of marine engines, both gasoline and diesel, suggest that engine, generator, and transmission oils and filters be changed before the boat is put in storage for the winter. This is typically the case even if you haven’t attained the suggested hours for an oil change.

There are good reasons for this. Running your mains during the boating season causes acid and water to accumulate in the oil. This is especially true if you do a lot of slow-speed operation. Changing the oil in the fall removes the acid, which can cause corrosion and other damage to the precision surfaces inside the engine. It also removes the water, which can separate from the oil and freeze and cause corrosion. Let acid and water remain and you risk premature engine, oil cooler, and transmission failure.

Before you change your oil always warm up your engines. This will mix any acid and water with the old oil so that they are removed when the oil is pumped out.

And remember: Preventive maintenance is the key to engine longevity. To best protect the interior surfaces of an engine during winter layup, always start and run it for a few minutes after an oil change—this circulates fresh protective lubricant. And always use high-quality lubricants that meet or exceed the specifications recommended by the engine manufacturer.

Professor Diesel is Larry Berlin, director of Mack Boring’s Training Services division.

This article originally appeared in the January 2012 issue of Power & Motoryacht magazine.