I'm trying to source 15w40 oil for a marine engine (indmar monsoon 325 - a marinized version of a centerbolt SBC), however I'm not finding any conventional oils at most automotive stores near me. They all have a selection of 15w40 oil for diesel engines, including in big drums which is fantastic for me as this is a rebuild and will be going through many oil changes in the first few hours of its life.
The oils I am seeing for diesel do meet gasoline engine specifications as well, however they are marketed primarily for diesel engines.
Is there a fundamental difference between these oils? I know I need conventional oil for break-in, my concern is that diesel oils will be mostly synthetic as they burn hotter, whereas a marine engine on the opposite end of the spectrum only runs at 160 degrees.