Mercury - 25W-40 4-Stroke Marine Engine Oil, 1 Quart FourStroke Lubrication Series 25W-40 Marine Oil 8M0078619. Overview. There's a certain comfort in hearing your engine settle into a smooth idle after you launch. No rattles, no hesitation-just that steady, familiar hum that tells you everything is ready. Clean oil is a big part of that feeling, especially on a boat where heat, humidity, and salty air are always trying to wear things down. When you're counting on your motor to bring you back af