Mercury - Full Synthetic 4-Stroke ATV/UTV Engine Oil 5W-50 8M0149407. Overview. If you spend enough time riding backcountry trails or working long days on the ranch, you learn to appreciate the small things that keep your machine running strong. Good oil is one of them. There's a certain confidence that comes from knowing your engine will fire up without hesitation on a cold morning, or carry you through a long, dusty climb without stumbling. That's the reliability riders look for, and it's exac