Make sure you use a clean container or one previously used for new oil, to avoid contaminating your sample. It will give you a good baseline for future analysis. It might also turn up an issue with the engine that the survey missed. I know of one instance where the buyer did that and found antifreeze in the oil, indicating a cracked block. He was able to get money back from the seller. Other things can show up too. Do it immediately, because time delay after date of purchase works against you if there is a dispute.

If purchasing a boat I would always include engine oil analysis as part of the survey, and if bought off-season would have enough of a reserve held in escrow to cover a re-power until a sea trial AND oil analysis. If buying off season and the engine oil was already changed, it might be impossible to get a meaningful analysis, but you could negotiate a hold-back for let's say 6 or 12 engine hours, which might tell you something even if just trace amounts that are inconclusive. Oil analysis is cheap.