I've now checked and I think there might be too much oil in what I think is the reverse gear housing. The engine has two dipsticks, one in the middle section on the port side, the other in the starboard aft section of the engine, in what looks like the transmission area. In the latter the oil reading is higher than the flat section of the dipstick by about half an inch. Yet, this couldn't be the cause, could it, if the slick is appearing towards the end of an hour of motoring and not at startup?
Never having checked the oil before this (dumb of me, I know), I don't know if the oil level there is increasing, but it's clean yellow translucent oil - as I would expect if the oil has been recently changed, so I'm inclined to think it's not "growing oil" as I think then it would be dirty, milky, watery, or whatnot.
I didn't check the O-rings as, from what I can tell, I'd have to take things apart for that. I'm not quite ready to do that yet.
Cardiacpaul, thanks for that insight re: tiny amounts creating quite a slick. Actually, the sight of slick, any slick, did alarm me. This kind of slick is normal with boat engines?
Re: air block shutdown, the engine actually wasn't shut down when I noticed the slick, I like to leave the engine running in neutral until I have the boat securely tied up. It was as I was tying up and before shutdown that I noticed it.
From what I'm learning this unburnt fuel getting into the exhaust business seems like the most likely source of the slick. I say this because, to my (inexperienced) mind, that's the one place where oil/fuel and water mix, by design, as the cooling water goes through the engine. Everywhere else the oil and water are kept separate and mix only if there's damage or corrosion.
So that seems like the most logical place to start ruling things out. I guess checking the injectors and timing is my next step. Gonna research how to do that.