My experience is with Avgas burning air-cooled piston engines. Avgas is 110 octane with no ethanol so that is not a variable. Aircraft engine manufacturers provide a reccomended "Time Between Overhaul" (TBO). This is typically measured by running hours but there are also testing metrics that are checked at least once per year at the FAAMandated Annual Inspection.
Aircraft that are in regular use often see engines in good shape well after TBO. Conversely, those that sit for long periods of time often require overhaul in less than half TBO.
Example; Lycoming 540 wth 2,000 hour reccomended TBO in an aircraft that flys at least every other week may not need an overhaul for 2,500 hours. That same engine run only once every 2 months, may not make it past 1,000 hours.
I think this demonstrates that longevity depends, in large part, to how often the engine is run.
BTW
General Aviation Aircraft have a low point drain port that allows visual inspection of the contents that have settled at the lowest point in the system. It would be nice to have something similar for boats.