A diesel is a very different beast than a gas engine. It is a much higher compression engine and the oil gets dirty much faster.

Another personal story. My brother had a bad accident a couple of years ago and I went to Chicago to put his boat away that fall. He only changed the oil once a year and is the kind of sailor that would rather drift than motor. When I pumped the oil out it smelled terrible - it was actually rancid. I gave him a lecture on getting his engine hot and changing the oil twice a season. His engine still runs OK but not great. A replace/rebuild is in it's future.