I use 85wt gear oil in the outdrive, and the engine I use delo 400 diesel engine oil... it has superior protection over standard automotive engine oils, and is a better oil than semi-synthetics....
as far as full synthetic oils go, their main claim to fame is that they perform better at extreme high temps that some engines experience.... in marine engines that never exceed 200 degrees, there is no point in using them.... some people believe they offer something at the lower temperature levels, but this is false information given by some manufactures as a selling point.... yes, it offers good protection, but no better than a dino oil unless there are regularly sustained high temperatures.....
if using an automotive oil, I would recommend using a Zinc additive in the oil, which is what "break in oil" has in it, as the good lubricity properties of zinc has been removed from regular automotive oils, which are needed in older engines. modern engines with roller cams have less issues with the lack of lubricity that zinc offers...
if your engine never exceeds about 170 degrees, a 10-30 or 20-30 wt oil is good... the higher temp engines, (or worn engines) may benefit from a 15-40 oil, but offers little for a low temp engine like a smaller boat engine...