Lee, It seems your boat's engine has a very large sump, which is not uncommon for an industrial engine.  I would be curious to know the following 1-Engine displacement, rated maximum hp and RPMs, fuel consumption at max HP/RPM 2-RPMs you typically run at and fuel consumption at your usual running speed 3-Typical hours you run each year. 4-Amount of top up oil you add each year. 5-What are the engine manufacturer's guidelines on when to change the oil. I think your strategy is a good one for the size of your sump. I'd carefully watch TBN and % soot in your oil test results. All the best, Gary Hagstrom Iron River, Wi.