Major assumption
There's a major assumption here that when buying a boat and taking an oil sample for survey that the oil wasn't changed prior to the survey/oil sample.
As a rule, I don't take oil samples unless requested. Why? Most owner's who are savvy change their oil regularly and even if they aren't, will most likely change the oil prior to selling. This way the crankcase oil will look good to a prospective buyer and a semi-clueless surveyor. It's amazing how much deferred maintainance gets done prior to selling a boat!
I do inspect the crankcase oil and after being a marine engineer for 20 years and looking at thousands of gallons of oil I've learned what may contaminate oil. A simple smear test will reveal alot about oil - water, coolant and fuel contamination are high up there in problems. Of all the oil tests that I've done, rarely was there any metal wear problems that weren't detectable by taking the oil filter, cutting it open and having a look. That's were all the metal bits will collect after all. In addition, a magnetic drain plug is also helpful in collecting wayward metal bits. As for trivial amounts of metal in a test, you need trending analysis by taking samples at regular intervals, otherwise it's not tremendously helpful unless there are tell-tale issues (bad valve adjustment, blue smoke, strange noises, funky turbo, etc.), metal chunks visible or other problems. Changing your oil at least once a year or at regular intervals is the key!
In addition, looking around an engine room will tell me whether the owner has completed any regular maintenance. If I see the engine is covered in rust, the bilge full of crap (oil, rags, etc.), the Racors are full of crud and other basic items not taken care of, I can assure you the oil wasn't changed any time in the past decade either. Also, how easy it is to change the oil tells me whether it gets done regularly. If it's a real pain, I doubt it gets done on schedule.
Surveyor's prospective:
John M.