Does engine oil actually go bad from just sitting within an engine that's not getting use? I feel conventional motor oil has been sitting underground for millions of years prior to being extracted and refined. But I have a 1995 Mercruiser 350 (small block Chevy) marine engine with under 200 hours on it that hasn't had its oil changed since 2004. Am I doing significantly more damage to this engine by not changing the oil right now before running it this weekend?
Short answer .... NO !!
Now this is one of those seriously contentious topics that people will chime in from all walks of life, experience, and voodoo.
I will speak [yell] my piece, and put on my Nomex undies:
Oil goes "bad" from two things:
The long chains get broken down due to wear. But ask any tribologist with an ounce of morals, and they will tell you that modern engine oil is sufficient to last MANY thousands of miles, perhaps 10-20K. (I get that you speak hours - I'm too lazy to translate that to knots.) And that's good old dinosaur juice. Modern synthetic compounds have wonderful properties, and might last twice as long as the organic stuff. In fact, with the exception of point #2, the right synthetic might last 50,000 miles. (x 0.868976 knots)
BUT BUT BUT.... Oil gets dirty. Little particles of soot and combustion products and small insects and detris and debris and schmutz and water accumulate in your oil storage "area". A really good oil filter [e.g. Oberg] might filter down to 5 micron or less, but require frequent cleanings. Hence you only see this type of filter in racing and special applications.
2A. So what? Some of that dirt is abrasive. None of it is helpful. It is circulated under pressure. Now imagine that your very expensive synthetic oil is circulating little tiny abrasive bits all around those critical lubricated interface points. Doesn't matter how much the "excipient" [big word] costs, what you have created is a cutting fluid - no matter how expensive. The size of the little cutter buggers is only limited by your oil filter. And your standard oil filter is a balance between longevity and particle size.
So So So?
Oil gets dirty. (Did I yell that yet?) AND OIL GETS DIRTY AT THE SAME RATE WHETHER IT'S USED "Wolfshead" OIL FILTERED THROUGH WONDERBREAD, OR ULTRA-SYNTHETIC DERIVED FROM SIBERIAN ANT GLANDS AND COSTS $5000/ounce.
(whew. okay, I took a breath)
So... to "answer your actual question" [cough] The one thing that does NOT make oil dirty is non-use. Unless you are parked in a desert monsoon or something. That's not even a thing. It just doesn't happen.
I know such statements are gonna get me some negative attention. But to summarize, oil does not "spoil", and non-use certainly doesn't sever the long chain molecules.
Yes, moisture that accumulates from combustion by-products should be vaporized off once in a while, but oil is not hygroscopic, and extended storage will not add bad things. The only issue is "rundown", or long term storage having the oil seep out of critical wear points... which makes that first "cold start" of the season perhaps detrimental. However, again, this is NOT a function of the oil, oil type, or change frequency.
[So could you stop yelling and have a point?]
The bottom line is that you will be much better served starting your boat motor every month for a minute or two in drydock. Even without cooling, if your unit draws seawater. This will "re-wet" the important surfaces with oil and prevent terrible "cold-start" damage. If you can't do that, I'd suggest a oil pressure revival by removing all the spark plugs and cranking long enough to achieve oil pressure. Or squirt some oil in the cylinders before start-up. But EITHER way, it's not the age of the oil, it's the lack of oil in critical places.
The marketing idea that "oil spoils! MUST replace every six months!" is a bit of marketing horsefeathers that makes me rant like I'm doing now. Oil doesn't get moldy, it doesn't spoil, and it doesn't loose lubricity from non-use. Just like you mentioned, these are very stable molecules that can exist for millenia (the very reason why you shouldn't pour waste oil on the weeds).
But from 2004? I'd put fresh oil in it just for the exercise, and crank with spark plugs out and a few squirts in each cylinder. Again, it's not the age of the oil, it's the fact all the important bits have gone "dry" in 13 years.
On Edit: [I realized later you said the oil had not been changed since 2004, not that you haven't run the boat since then. All the better.]
YMMV. Best of luck...