An engine unit is equipped with a cylinder head and a crank case in which an oil feed pump and a scavenge pump are disposed. A lubricating and cooling system for the engine unit comprises an oil tank constituted by a hollow main tube, a hollow down tube, a hollow bridge tube connecting the main tube and the down tube so as to communicate with each other through oil ports and frame members covering a space defined by the respective tubes. A feed pipe connects the down tube to the feed pump, a scavenge pipe connects the oil tank to the scavenge pump, and a flowout pipe directly or indirectly connects the cylinder head to the oil tank. The oil pots formed in the main tube, the down tube and the bridge tube have opening sizes selected to effectively separate air from the engine oil during the circulation thereof.