Mercury - 10W-40 4-Stroke Marine Engine Oil, 1 Gallon FourStroke Lubrication Series Mineral Base Oil 8M0148477. Overview. Oil changes aren't the glamorous part of boating, but they're the part that keeps the whole story going. When you're miles from the ramp and the water turns unpredictable, you want to know your engine is protected on the inside, not just rinsed clean on the outside. Good marine oil gives you that quiet confidence-like the motor is running smooth, steady, and cared for, even w