Oil change with full flush?

I ve always used Rotella and never noticed any pressure drop. I put 4000 hours on a pair of 3412Es using rotella, boat sold with 5400 hours and passed survey/oil analysis without issues. Just a puff of smoke on cold starts

Exactly. No engine manufacturer makes their own oil.
All the oil is made by someone else to their specifications and private labeled.
And I’m sure that goes out to bid so it probably changes over the years on who’s making their private label oil.
Could be Mobil one year , Shell or others the next .
Personally I use Valvoline 40 weight
Which I get from Napa under their private label.
It’s been doing just fine for the past few years for me .

Each engine manufacturer does have the oil blended to their specifications. Synthetic blends can vary greatly in how much synthetic is actually in the oil. The CAT oil is the same price as Mobil or others, so I just use the manufacturers.........generally there isn't hardly any difference in price when it comes to oil. Cat and MAN have been using Mobil for decades, so it's not price shopping. As long as it meets the engine manufacturers required specs, you should be ok.

Aren't the new guidelines directed at new engines. My engines are nearly 25 years old. Mobil Delvac for me. Oil changes are cheap. Pistons aren't.

Been using Rotella in Cat engines for years without an issue. Also read that there is no noticeable difference in wear between conventional oil brands if changed as per mfg'r requirements.

No doubt you , given your profession, you should use the factory labeled oil for all your work an nothing else unless the owner tells you other wise . CYA. I would be doing the same thing if I were you.

Ole Cleanslate on the other hand has to watch his money very carefully , so I will save where I think it's rite. BUT I have professed to be a Michael Stivic at time her on Y.F. and get myself into boat trouble by being to cheap and not so smart.

Not the most reassuring sort of previous owner to buy a boat from...

Yep. #11 " If you switch to synthetic that would be a reason to flush the motor as you'll want to get the old oil out. They don't blend." Personally I prefer synthetic. More expensive but not nearly so much as switching back and forth. Now your big question is was everything flushed and changed? Might want to do an oil analysis on all before any more changes.