An oil return passage for returning oil from a cylinder head to an oil pan is arranged along the downward first side surface of an engine that is mounted in a vehicle such that a center axis of a cylinder thereof is slanted with respect to a vertical direction of the vehicle. An oil return extension pipe is coupled to an outlet end of the oil return passage, and extends toward a second side surface of the engine that is opposite from the first side surface on which the oil return passage is provided. Thus, engine oil can be reliably returned to the oil pan from the cylinder head of the engine even when the vehicle is experiencing high acceleration in a direction that tends to cause backflow of the oil through the engine oil return system.