Mercury - Synthetic Blend 4-Stroke 25W-40 Marine Oil Quart Engine Protection 92-8M0078629. Overview. Most of us end up using this oil because it just behaves the way you expect it to. It's got a thicker, steady feel when you pour it, and once the engine warms up, it settles in nicely. I've run it on hot days, long no-wake stretches, quick hops between spots-you name it-and the motor stays calm. It's the kind of thing you keep on the shelf because it never gives you a reason to switch.. Key Featu