Whenever asked about the best marine oils, I have a simple answer: Buy the same brand as your engine. Builders such as Cummins, Detroit Diesel, Evinrude, MerCruiser, Suzuki, Tohatsu, Volvo Penta, Yamaha and Yanmar have vested interests in keeping their engines running strong, particularly during the factory-warranty period, which might stretch to six years. So it makes sense that the factories formulate oils to keep their power trains in the best possible condition. Availability of factory brands is often limited to authorized dealers, and these products usually carry a premium price. Call it cheap insurance.