Mercury - Mercury 25W-40 4-Stroke Marine Engine Oil Marine Engine Oil Gallon 8M0078628. Overview. There's a certain calm that settles in when you know your engine is protected, no matter what the day throws at you. Anyone who spends time on the water understands that confidence is everything - the steady hum at idle, the strong push onto plane, the long ride home after sunset. Mercury 25W-40 4-Stroke Marine Engine Oil, part number 8M0078628, is one of those behind-the-scenes essentials that keep