Motor oil/additive packages are tweaked frequently for any number of reasons, more recently some of the changes are to modify or reduce additives that have a negative impact on modern emissions control systems; not sure how much of that is applicable to a marine engine.
but as california jobcase said, it’s fine to use a newer revision of oil than your engine calls for, but you really want to make sure it has the right certifications. the problem you’re running into is that 15W40 is a common viscosity for older diesel engines, but not so common for gas engines. The older oils I used in my diesel trucks were dual rated for gas and diesel engines (back then the standards were SF/CD.) but with the complexity of modern diesel emissions controls, I think that there are more commonly separate oils with no S* rating but only a C* rating.
you might have to track down a “marine” motor oil to get what you need, and pay through the nose for it.