Oil tests show signs of wear by looking for metals and crud, such as...
Chromium from piston rings, cylinder liners and exhaust valves
Nickel from crankshafts, camshafts and valves
Aluminum from pistons and bearings
Copper from bushings and oil coolers
Lead and tin from the crankshaft main bearings
Titanium from bearings, valves, struts and connecting rod pins
Silicon, a “contaminant metal” indicating dirt. It’s more common in samples from land vehicles, but it can appear in a marine engine with a clogged air intake.
Sodium and potassium from the cooling system. A corrosion pinhole can admit antifreeze, diluting the oil. Most gas and diesel inboards in salt water have closed-loop freshwater systems, but they still have components through which raw water flows, like heat exchangers and aftercoolers.
Fuel from blow-by. It dilutes oil, triggering damage when moving parts scuff against one another.
Soot from incomplete combustion. It indicates dirty or worn injectors, improper fuel mixture, inadequate air intake flow, blow-by or an obstructed exhaust system. Diagnosing the problem and correcting it improves performance and lowers fuel consumption.