I would consider the National Marine Manufacturers Association (NMMA), a long standing and well respected organization, to be the authority on the needs of marine engines. I would be inclined to trust their recommendations over Internet opinions. The NMMA states the following in their Certification Procedure Manual for FC-W® and FC-W Catalyst Compatible® Four-Stroke Cycle, Water-Cooled Gasoline Engine Lubricants:
"Recently introduced, larger 4 and 6 cylinder four-stroke cycle outboard engines are now taking the place of some conventional two-stroke cycle outboard engines. The duty cycles of marine engines have always discriminated these engines from over-the-road light and heavy duty automotive engines. In particular, the lubrication demands of marine engines are unique. High loads, high speeds, and exposure to high levels of humidity and salt water are just a few of the unique characteristics of an engine in the marine environment. Aware of the particular lubrication requirements of four-stroke cycle engines in the marine market and their increasing use, the National Marine Manufacturers Association Oil Certification Committee developed the four-stroke cycle engine oil specification, NMMA FC-W®."
Clearly the industry technical authority on four-stroke marine engines believes the lubrication demands of these engines are unique and different from that of light and heavy duty automotive engines, and accordingly have developed a set of tests and specifications specific to the needs of marine four-stroke engines. That's good enough for me, even without knowing the details of how the marine formulations are different. Therefore I would place my trust in the NMMA and look for certification against FC-W.
"Recently introduced, larger 4 and 6 cylinder four-stroke cycle outboard engines are now taking the place of some conventional two-stroke cycle outboard engines. The duty cycles of marine engines have always discriminated these engines from over-the-road light and heavy duty automotive engines. In particular, the lubrication demands of marine engines are unique. High loads, high speeds, and exposure to high levels of humidity and salt water are just a few of the unique characteristics of an engine in the marine environment. Aware of the particular lubrication requirements of four-stroke cycle engines in the marine market and their increasing use, the National Marine Manufacturers Association Oil Certification Committee developed the four-stroke cycle engine oil specification, NMMA FC-W®."
Clearly the industry technical authority on four-stroke marine engines believes the lubrication demands of these engines are unique and different from that of light and heavy duty automotive engines, and accordingly have developed a set of tests and specifications specific to the needs of marine four-stroke engines. That's good enough for me, even without knowing the details of how the marine formulations are different. Therefore I would place my trust in the NMMA and look for certification against FC-W.