Mercury - 15W-40 Heavy Duty Diesel Engine Oil 858042K01. Overview. Marine diesel engines live a tougher life than most - long hours, heavy loads, and salt air that doesn't forgive a thing. That is why Mercury 15W-40 Heavy Duty Diesel Engine Oil exists. It is made for boaters who understand that reliability isn't optional. Every turn of the crank, every trip offshore, this oil is there - protecting, cooling, and cleaning. It's the difference between worrying about your engine and trusting it comp