A transmission capable of being assembled as a coolerless transmission and a transmission having an oil cooler includes a housing and a lubrication system. The oil passage system includes an oil passage having an oil outlet opening extending out of the housing and an oil inlet opening extending into the housing. A bypass flow passage is disposed in the housing in communication with the oil outlet opening and the oil inlet opening. In a coolerless configuration a pair of plugs are inserted in the oil outlet opening and the oil inlet opening, respectively, to close off the oil outlet opening and the oil inlet opening so that oil flows from the oil outlet opening to the oil inlet opening through the bypass flow passage. In a transmission configuration having an oil cooler, an exterior oil cooler is connected to the oil outlet opening and the oil inlet opening.