I generally change my oil in the fall just before haulout and then again in the spring right after launch. In the fall I use Walmart brant diesel rated oil. In the spring I use Texaco Ursa or Chevron Delo 400. Probably the same oil actually since Chevron and Texaco are the same company

I don't think there is any difference between oils from the major suppliers. I generally go with the one that I can get both fgallons and quarts for since my engine takes 5.5 quarts. Lately that has been Texaco Ursa. I also use the 15W40 because it is better for those cool spring and fall starts. That is what most of the lobster boats use here