Engine oils are lubricants that are used to reduce friction and wear between moving parts in internal combustion engines. They help to keep engines running smoothly and efficiently, and also help to keep them clean by preventing the build-up of harmful deposits. Engine oils are classified into different types, such as conventional, synthetic, and high-mileage, based on their formulation and properties. The type of oil used depends on various factors, including the type of engine, driving conditions, and manufacturer's recommendations.