Synthetic oil is a man-made lubricant composed of petroleum components and other chemical compounds, although it typically uses distilled crude oil as its base. It's often used in engines as a substitute for conventional oil, but it has some clear advantages over it. For starters, it is far more stable than conventional oil and is less likely to acidify or oxidize. This means that it will last far longer and provide your engine with greater protection; most synthetic oils are rated to last between 10,000 and 15,000 miles before they need to be changed.