Mercury - Oil Change Kit, 15-20HP FourStroke, 1 Liter Maintenance 8M0081910. Overview. These little 15 and 20HP Mercury FourStrokes don't ask for much, but they do appreciate fresh oil. They spend their days pushing skiffs, dinghies, and small fishing rigs-idling around, getting hammered by heat, then jumping straight into work again. When the oil starts getting worn, the motor feels a bit rougher, a little slower to respond, and just not as "eager." This kit keeps the whole job simple: the righ