before they change the engine oil, get an oil sample done.
I've found that most marinas will give you the "doe-eyes-in-the-headlamp look". They are available from ANY construction equipment dealer, or heavy truck dealership that does service. The cost is minimal, about 15.00.
The engine oil sample will tell you many things. Fuel dilution, ash content, what type and how much of any metals that are wandering around in your engine.
For example, Chromium - piston rings. Lead/tin - main/rod bearing wear, Boron- cyl wall wear, Butyl, gaskets/seals/o-rings.
2 weeks ago, I did a survey on an almost perfect 38ft Scarab with twin 502 Merc's with less than 300 hours on the engines. The oil sample revealed severe fuel dilution (leaking injectors @ 140.00 each x 16) and more importantly Copper, indicating crankshaft thrust washers were going south quickly, also hurting the crank and main&rod bearings. As I write this, one engine is being replaced and the other is being rebuilt at the dealers expense, not the prospective owner. The estimated repair bill is 38,500.00. On a 95k boat.
Little guys like UPS, Roadway Fed-ex, Caterpiller, Cummins, John Deere and Detroit Diesel use oil samples as a matter of policy.
We are not primarily on earth to see through one another, but to see one another through
Some people are like slinkies: not really good for anything... but you can't help laughing when you push them down the stairs