Need Advice Please

Mercruiser 454 Gen 4 on a FN noticed this summer on a few short runs I had to add some oil . Yesterday I traveled 90 miles, and when I got back to the landing oil pressure started dropped to drop below 10 psi...

Normal 70 cold
40-45 hot
65 running at 3k rpm’s

Anyway, I get home to find I have little or no oil and no leaks .. boat ran great .
Raw water temp 140-145 ( 140 Tstat )

Oil temp 220-228 (full temp )

Pulled the four easiest spark plugs on the corners and the starboard plugs had some dry soot almost like carbon .. plugs were brand new …. Hoping it’s either oil cooler but doubt it due to soot , so hopefully valve stem seals ?

According to this linked thread from last year it's a low hour used early 454 that he had some oil related questions about 

After pulling all the spark plugs all but 2 have excessive carbon buildup. Could this carbon build up get on the rings, causing oil to burn thinking of soaking them overnight with marvel mystery, oil, or something along those lines?

Since it doesn't sound like you were leaving a huge mosquito killin' cloud of smoke in the exhaust and you used let's say, at least a gallon of oil in 90 miles, that oil cooler is looking more and more suspicious

Update

Ended up doing a top end engine clean to remove. All the carbon ended up doing it four times. Never seen so much carbon in my life. I had gotten this motor from a guy that lived in the Great Lakes. Had it rebuilt and only put a couple of hundred hours on it all up north with no thermostat.
Change my oil and filter a couple of times. Don’t see any blow by at all. No signs of oil in the water out of the exhaust only steam running on the hose, and definitely no water in the engine room... Oil pressure does seem a little high. I’m running straight 40. With cold engine and giving a slight rev to 2500 RPMs. All pressure will go up to about 90 pounds for a couple of seconds. Then immediately come back down to the 65 to 70 really starting to believe that I had excessive carbon buildup, making the piston ring stick and not be able to expand properly.

Steam only out the exhaust???   IS the boat on the water or sitting on the trailer?   In the water that may be normal, on the trailer that would be a problem with no water flow or not enough reaching the exhaust.   I have never seen a 454 carbon stick the piston rings, Honda engines yep, some Toyota engines but not one 454.