Returning engine oil to the primary oil sump from outboard locations in a flat engine can be a challenge. In the present disclosure, the pressure fluctuations occurring at the underside of the piston, due to reciprocation of the piston, are used to pump the oil from outboard locations to the primary oil sump. To prevent backflow of oil during periods of negative pressure difference, a check valve is placed in the oil drain passage. The check valve allows flow from the outboard location to the primary oil sump and no flow from the primary oil sump to the outboard location by opening and closing as driven by pressure fluctuations due to piston reciprocation. Such a check valve also prevents backflow of the oil to one of the outboard locations when the engine is tilted.