A turbofan engine having a heat exchange system for use in lubricating systems for turbofan engine equipment in which a lubricant is utilized in providing selected operations thereof, the heat exchange system providing air and lubricant heat exchanges to cool the lubricant at selectively variable rates in fan airstreams occurring within an engine fan duct that extends between the turbofan engine fan and a fan duct nozzle, the system comprising: a cooling duct extending through at least a portion of a stream structure that is positioned in the fan airstreams occurring during use, the cooling duct having an entrance opening on an upstream side and an exit opening on a downstream side; a moveable entrance flap disposed on the upstream side of the cooling duct such that the moveable entrance flap covers at least a portion of the entrance opening when in a closed position and permits flow of air through the cooling duct when in an open position; a moveable exit flap disposed on the downstream side of the cooling duct such that the moveable exit flap covers at least a portion of the exit opening when in a closed position and permits flow of air through the cooling duct when in an open position; and a heat exchanger core provided in the cooling duct between the entrance and exit openings thereof and having a plurality of passageway structures therein about which air can pass in flowing therethrough with the passageway structures being coupled to an input conduit at one end thereof and coupled to an output conduit at an opposite end thereof to enable the lubricant to be provided, and removed from, interiors of the passageway structures through interiors of the input and output conduits; wherein said stream structure is a fan duct lower bifurcation structure positioned between the bottom of the wall of an engine nacelle and the bottom of the wall of an outer fairing of an engine pod for the turbofan engine a cooling duct extending through at least a portion of a stream structure that is positioned in the fan airstreams occurring during use, the cooling duct having an entrance opening on an upstream side and an exit opening on a downstream side; a moveable entrance flap disposed on the upstream side of the cooling duct such that the moveable entrance flap covers at least a portion of the entrance opening when in a closed position and permits flow of air through the cooling duct when in an open position; a moveable exit flap disposed on the downstream side of the cooling duct such that the moveable exit flap covers at least a portion of the exit opening when in a closed position and permits flow of air through the cooling duct when in an open position; and a heat exchanger core provided in the cooling duct between the entrance and exit openings thereof and having a plurality of passageway structures therein about which air can pass in flowing therethrough with the passageway structures being coupled to an input conduit at one end thereof and coupled to an output conduit at an opposite end thereof to enable the lubricant to be provided, and removed from, interiors of the passageway structures through interiors of the input and output conduits; wherein said stream structure is a fan duct lower bifurcation structure positioned between the bottom of the wall of an engine nacelle and the bottom of the wall of an outer fairing of an engine pod for the turbofan engine