Mercury - 10W-30 4-Stroke Mineral Marine Engine Oil, 1 Gallon FourStroke Lubrication Series Mineral Base Oil 10W-30 8M0078626. Overview. A good day on the water starts with an engine you don't have to think about. Turn the key, let it settle into a smooth idle, and you can focus on the fun stuff-finding fish, chasing sunsets, or just getting the family to the sandbar without a second worry. The truth is, marine engines live in a world that's always trying to wear them down: humidity, salt air, l