For an older engine the conventional logic is to use a single weight oil, such as 30 weight. However,I believe the first thing to consider is where your boat is used. If it is in the Northwest, such as mine, where the water temperature is cold year around, then a 15-30 is probably OK. But then it depends on how many hours your engine has (thus wear and looseness) and also how you run it (if you run the engine hard for long periods of time). However, when I had a boat in Mexico and Tahiti for a few years, where the water never was much below 70 and often in the 80s year around, I always used 30 weight.
Be sure to use a heavy duty oil such as Rotella (Shell) or Delo 400 (Chevron). I had a friend who used to sell Amsoil and swore by it (but then he sold the stuff). Diesels require the heavier duty oil because of the extra strains caused by the higher compression ratios.
Also I don't believe in adding any additives to my oil as I think the oil company engineers probably have a pretty good idea how to formulate the stuff in the first place.