Car engines don't do tens of of thousands of kilometres before requiring a workshop visit, as you seem to think. Although a lot of people don't know it most engine oils are only good for 10k or 6 months, what ever comes 1st. The lubricating properties in the oil are no good after 6 months, regardles of klms, this is one of the hardest things to convince people of. It doesn't matter if the car sits in your garage for 6 months, after that the oil is cactus. On what knowledge are you advising sammy to go double the amount of hours the service schedule says before he needs a service, it's easy to advise someone not to worry about the service schedule when it's their motor and not yours. I used to be a mechanic and I change my car oil every 5k or 3 months. I bet You've never seen the inside of a motor that has had regular oil changes compared to one that has been basicly neglected, or let go for 20k or so, time after time. Let me tell you, they're filthy, dry, hard, baked on oil all over the place, bloody horrible, but everyone thinks, "it still goes so why should I service it?" Besides that, a car engine is under a lot less stress than an outboard so I would be enclined to follow the service schedule, basicly, if you bought a 4 stroke, you paid all that $$$ for it, you have to maintain it.
Just my 2cent worth!!!