The "secret" to prolonging the life of a four-stroke marine engine is common-sense simple: Change the crankcase oil regularly, keep the ignition system in tune, and monitor the cooling system. If you follow these three rules, your outboard motor, stern drive or inboard gas or diesel will grow old gracefully. On the other hand, should you neglect to maintain these systems the odds are good that your engine's health will suffer, and its lifespan will consequently be shortened. For more, please check out the Marine Engine Digest article.