Abstract: An arrangement and structure for an oil pan system to permit mounting a vertical in-line engine in a vehicle in a horizontal or nearly horizontal attitude includes a cylinder block of the engine having an opening on the side wall thereof, a cover attached to the cylinder block where an ordinary oil pan is usually mounted, an adapter plate fastened to the lower side wall of the cylinder block and bottom of the cover, and an oil pan attached to the lower surface of the adapter plate. The adapter plate allows the oil pan to extend and always remain below the cylinder block, so that the return paths from the lubricated portions of the engine to the oil pan are shortened.