Mercury - Quicksilver 10W-40 Full Synthetic Off-Road Engine Oil 4-Stroke Protection Full Synthetic Formula 8M0128395. Overview. If you spend your free time on trails, dirt roads, or tucked deep into the woods, you already know how unpredictable off-road riding can be. One minute you're cruising across hardpack, the next you're crawling through mud, rocks, or steep switchbacks. In those moments, you need an engine that responds instantly, stays cool under pressure, and never fights against the co