Mercury - 25W-40 4-Stroke Marine Engine Oil, 1 Quart FourStroke Lubrication Series Marine Engine Oil 8M0078627. Overview. Marine engines live a harder life than most people realize. One day it's a long, hot run across open water, the next it's slow idling in a tight channel with humid air hanging over the stern. That mix of heat, moisture, and load is exactly why the right oil matters. When you're using a true marine oil, you can feel the difference in how steady the engine sounds and how confid