Let me make a comment as someone quite familiar with modern lubricant technology. Modern engine lubricants are deeply misunderstood. Lubricants are highly technical and the development costs are tens of millions of dollars. Very few outside of the additive and lubricant business really understand how things have changed over the years and how very complex modern formulations are. Simple measures like TBN alone are not particularly meaningful in determining oil life. Many people have opinions about engine oil but most don't have much actual knowledge. This includes mechanics who know engines but not lubricants.
The first question as to when to change oil. As to the initial question from Silentknight as to changing oil now vs. the spring. It makes no difference, do it when it makes logistical sense.
In terms of leaving 'used oil' in an engine over the winter, i.e. changing in the fall vs. spring. This is mostly driven by convenience. The idea that the engine will sit and corrode over the winter unless new oil is present isn't true unless you have some other problem like lots of water in the crankcase. Whether to change every year or not really depends on how many hours the engine ran. If you ran several hundred hours then yes, change at the end of the season. If you ran a low number of hours then it can go another year. One thing I don't recommend is leaving the same oil in for years. But I will admit that isn't scientific and oil is quite stable but its also inexpensive so even if you run low hours change it at least every couple years.
Having analyzed millions of miles of field test data from trucks and cars, I don't find TBN (either ASTM D2896 or D4739) to be very definitive. I know the oil analysis labs put flags in at arbitrary levels but these aren't definitive. TAN (total acid number ASTM D664) also isn't definitive but the combination of TAN with TBN at least tells you when the base reserve is nearing its end (sometimes called TBN/TAN crossover point). However, I've seen oils run with low TBN but no indication that Cu, Pb, or other corrosion metals were increasing. All oils drop TBN fairly rapidly at the start. This is because much of the BN in modern oils is 'soft', i.e. derived from the nitrogen in the ashless dispersant and diphenylamine antioxidants as well as other chemicals. So called 'hard' base from metallic detergents is about the same in all modern CK4 oils because the amount of total sulfated ash is limited to about 1%. Modern engines do not appreciate high detergent oils due to the tight crownlands and high ring location (for emissions).
Many marine engines are older models where current lubricants far exceed the performance needs of these engines. Modern oils have a longer life because they have much more performance additives and all modern oils use hydrocracked base oils (essentially a synthesized base oil). Chevron alludes to this by labeling their oils as "IsoSyn", but all the current lubricants use such base oils.
While I have an affinity for Chevron, I would not hesitate to recommend similar products from Shell, ExxonMobil, Total, BP, or other major companies. OEM branded oils (Caterpillar, Deere, Cummins) are also good though not necessarily better than the major branded products. I do not recommend boutique oils from small companies. The big companies have the resources and brand reputation to stand behind their products. Qualifying an API CK-4 product costs $10-20 million dollars, so it is a big buck process. R&D to develop the additives, the formulations, and base oils is in the hundreds of millions, every year.
