For years, I think close to 15 years, I have used Blackstone for my truck and tractor engines. If I had a boat, I would send the sample to them. The sample cost is around $35 if you include the TBN test. TBN is a measurement of how much of the oil additive is left in the oil. You don't want it to be zero.
The Blackstone report has a list of the previous 5 or 6 sample reports, and includes and average of the tested results from your engine as well as all of the other engines of that type they test. Very useful to figure out if the numbers in your sample are a problem or not. Seeming the numbers over time is important.
I don't think Blackstone cares about the engine at all. They are also pretty agnostic on the oil to use. Just use what the manual indicates.
The age of the oil will not affect the tests. I have sent them oil that had spent a couple of years in the engine with low hours. The report looked good. I assume in your case you are curious about the low hours but the age of the oil so get a test done.
My last truck sample was at 14,995 hours which is three times over the change period recommended by the manual.
Now, I do run the oil longer than the manual states but this was a few thousand hours more than I wanted to put on the oil but work was such I could not change the oil.
The oil was a year old as well.
The oil was fine, the TBN was still at 5.5ish, so in theory, I could have run a bit longer on the oil. Which is good to know but I don't really want to push it that much.
Later,
Dan