FR, Not sure if you know but synthetic lube/oil doesn't get like mayonnaise when mixed with water.
Well--- About a month after selling our Chaparral 260ssi (thru a broker), I spied it coming up the launch ramp at our harbor. I introduced myself to the new owners, explained I had a couple of parts to give them (short piece of water tank line and a manual), and asked them how they like the boat-? Guy says "we LOVE it, couldn't be happier, I just thought it would be faster..."
"can't even get it up to 40"...
Umm, you mean 50 maybe? This thing would cruise at 37mph at 3300 rpm, never had ANY trouble hitting 40...!
While chatting his wife was pulling it toward the parking slots, and when I turned and looked at the back of the boat my heart skipped a beat.
--there's why it won't go 40 I sez, while snow white oil was sadly oozing out of the dipstick hole of the DP drive, OMG... Guy said the broker changed out the engine oil and gear lube- full synth on both- before they took delivery. This was their 4th trip out. I told him he needs to have a stern talk with the broker about the missing dipstick and what to do about the drive. Haven't seen them since.
The 'mayonnaise' wasn't the consistency of putty like I've seen normal oil do, it still had some 'run' to it but not much.
(and it DID cruise 37 @ 3300, most 'relaxed' boat I ever owned!)