Mercury – 15W-40 Heavy Duty Diesel Engine Oil | 858042K01Mercury 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 doesnt forgive a thing. That is why Mercury 15W 40 Heavy Duty Diesel Engine Oil exists. It is made for boaters who understand that reliability isnt optional. Every turn of the crank, every trip offshore, this oil is there protecting, cooling, and cleaning. Its the difference between worrying