Mercury - 25W-40 4-Stroke Marine Engine Oil, 1 Quart (6-Pack) FourStroke Lubrication Series Marine Engine Oil 8M0078627. Overview. Marine engines live a tougher life than most people realize. One day it's a long, hot run across open water, the next it's slow idling in a no-wake zone with humid air hanging around the motor. Heat, load, moisture-it all adds up. That's why using the right oil matters more on a boat than just about anywhere else.. This Mercury 25W-40 4-Stroke Marine Engine Oil comes