Shell Rotella 550019858 comes up as SAE 40 oil,most likely for the old Detroit Diesel 2-stroke engines.I had six of them 30 years ago.They spec'd 30 winter and 40 summer.Ah,the good old days!There may be a 10W40 diesel oil,but I've never seen it and I'm a dinosaur.Detroit Diesels were popular in marine applications as well,since you're in Maine,it's most likely for the fisherman.