Mercury - 25W-40 Synthetic Blend Oil Four-Stroke Engine Protection 8M0078623. Overview. This 25W-40 synthetic blend is the kind of oil most boaters keep on the shelf because it helps a four-stroke run smoother and last longer. It handles those long days when the engine stays warm for hours, and it doesn't break down as quickly as lighter blends. Fresh oil like this usually quiets the motor a bit and gives it an easier, more relaxed feel. It's a steady choice for anyone doing routine service on a