Tom, your comment: "Talk about durability, I'd hate to be the guy who tossed these motors out when they reached 3,000 hours" rings home LOUD and CLEAR.
People do the darndest things when they see a little rust, have to replace an impeller and heaven FORBID change plugs or wires! Most of the time if you keep good oil in a motor the things that really go wrong are the cheap external ancillary things, like pumps, consumable ignition components, carbs, etc., and not the internal equipment. In the case of the 534 Super Duty, one with 3000 hours would be just broken in, and I shudder to think someone out there may have actually made the decison to haul them based on what they might have been told at the marina. It reminds me of the time I heard a man passed away leaving his beloved Chris Craft with twin 427 engines behind. His widow was then told by the marina that she should have "those old engines" replaced, which she did. I'll bet the guy was rolling over in his grave, and no doubt was (or is now) waiting to see that mechanic in person, lol. Many times when a motor is not running properly, sending out smoke, unburnt fuel smell, etc., it can be traced to bad valve seals (rubber replacement easy to do), perhaps a fouled plug causing incomplete combustion, a bad carb, perhaps some pinging due to clogged fuel filter, and I fear that some people begin thinking they need to spend $35,000 or more to go through a total repowering. Yes motors do wear out, but people's nerves wear out first. In this case it sometimes comes down to whether or not the owner will get himself professional help, spend a little money on a decent tune up (or do it himself naturally), go ahead buy a new coil for crying out loud, and get things right...........or reach for his wallet and spend $40,000 or more. One of the purposes of this forum is to help people avoid the latter.