Your marine diesel, if you ran it constantly would probably make the difference between a road engine oil not a big deal -- but when you just start the engine (cold) and run for a little bit, then shut it down. etc., etc. most of the normal wear-and-tear is in the starting. Also the engine may not come-up to temp and is consequently more subject to sulfuric acid issues on some of the parts.

i wouldn't 5 w/40 unless you were in the very cold country. the 10w40 would seem to make sense to me. The 15w40 probably won't kill it.