This continually recurring thread will likely be voted as the subject most often discussed and rehashed on this (and probably every other automotive) forum.
Larry Delor, the average recommended oil change interval on pleasure craft is every 25 to 40 hours. The actual average boat owner probably changes theirs between every 50 to 75 hours. Like every other internal combustion engine all of this is determined by whether the boat is towing a skier (or several skiers) at an average sustained 3500 to 5000 RPM, or cruising around a body of water, or just trolling.
The average recommended oil change interval for commercial diesel engines used in trucks and buses is between 10 and 15,000 miles... :p
Larry Bible, I'm too damn lazy to search the archives for one bit of information, when I can get it direct from "the Bible". What oil brand and viscosity do YOU use in your vehicles?
Incidentally, EVERY boat mechanic that I asked said that they use dipstick tube draining devices because they HAVE to. Not one of them told me that they ever use one for their own cars. However (do I hear a growing rumbling, grumbling sound from the "TopSider" users out there?), they all agreed with LB about performing a second suction the morning after removing all of the oil the day before, with the caveat that it should be done when the ambient air temperature is above 70°F to compensate for the oil having cooled off.
So far everyone agrees that:
1. You should change your engine oil regularly.
2. The oil should be changed while the engine is hot.
3. The filter should always be changed along with the oil.
4. The oil should be responsibly disposed of at a recycler.
5. You should never mix synthetics with dino-broth.
Correct? Any disputes on the above list? Good!
Everything else, IMHO, is open to opinion, and subject to variation based upon conditions present, type and duration of operation, and personal preferences.
But, I do still wonder what the deal is with equal oil change intervals being recommended for both gas and diesel engines. (I also wonder why diesel fuel costs more than premium 92 octane where I live, but thats another thread...) Shouldn't the change interval differ for two such radically different engine types? They do in commercial applications.