Thus, to account for this, the makeup of their oil is vastly different from automotive oil. Automotive engine oil isn't exactly the same across the board, so it's no surprise that comparing it to marine oil yields even more differences. The functional differences between marine and automotive engines, the conditions they're placed under, and the frequency at which they're used throughout the year play a major role in marine oil's different makeup . All that is to say that marine oil doesn't belong in automobiles and vice versa. One area where they do overlap, however, is that in the broad marine oil category, not every bottle contains the exact same stuff.