Use a quality motor oil and don't add a thing. It is a proven fact that a modern, properly formulated motor oil does not need to be conditioned, stabilized, treated, fortified, or anything else of the sort. Oil additives are only useful in separating you from your hard earned money. Now, oil additives don't necessarily do any harm, but they certainly do not provide any measurable benefits. That is why people get away with using them and believe they have found a magic "recipe". The recipe is already there in a quality motor oil. You don't need any high priced snake oil. Save your money. Boats are expensive enough.
If you want to do some reading and learn everything you will ever want to know about motor oil, Google "Bob Is The Oil Guy". It is a great site and if you spend some time there, you will completely re-think your ideas about lubricants.