Mercury - SAE 25W-40 Synthetic 4-Stroke Marine Oil, 1 Gallon Lubrication 92-8M0078630. Overview. Four-stroke marine engines don't get an easy break. They idle for long stretches, jump straight into hard throttle, and spend their whole life in heat and humidity. That's why the oil you pick matters. This 25W-40 synthetic blend is the kind of oil most mechanics stick with because it hangs onto its thickness, keeps metal parts coated on cold starts, and does a solid job fighting the corrosion that s