Motor oil is a viscous fluid lubricant utilized in internal combustion engines to minimize friction between moving parts—primarily the crankshaft, connecting rods, and piston assemblies. Formulated with base oils and performance additives, it cools engine components through heat transfer and removes combustion byproducts. Viscosity, measured by SAE grades, dictates oil flow at varying temperatures, while API service classifications define performance levels for gasoline and diesel engines. Detergents, dispersants, and anti-wear agents protect against sludge buildup, corrosion, and component damage. Modern formulations include synthetic and semi-synthetic options offering extended drain intervals and improved thermal stability.