A heat exchanger system comprising: a core structure (214, 314) at least partly defined by an elongate flow structure (216, 316), the elongate flow structure (216, 316) extending along an axis (218, 318) from a first end to a second end, wherein the elongate flow structure (216, 316) includes an outer surface (217, 317); an oil flow path (224, 324) that is internal to the elongate flow structure (216, 316) and that extends along the axis (218, 318) between the first end and the second end, the oil flow path (224, 324) configured to receive an oil flow; a fuel flow path (240, 340) that is internal to the elongate flow structure (216, 316) and that extends along the axis (218, 318) between the first end and the second end to be included in the core structure (214, 314), the fuel flow path (240, 340) configured to receive a fuel flow, the fuel flow path (240, 340) coupled to the oil flow path (224, 324) to allow the fuel flow to receive heat from the oil flow in the oil flow path (224, 324), the oil flow path (224, 324) and the fuel flow path (240, 340) being separated by a common wall within the elongate flow structure (216, 316), wherein in a cross section of the elongate flow structure (216, 316), the fuel flow path (240, 340) at least partly surrounds the oil flow path (224, 324) and wherein, in the cross section of the elongate flow structure (216, 316), the fuel flow path (240, 340) is disposed between the outer surface (217, 317) and the oil flow path (224, 324); a supplemental airflow path defined at least partly by the core structure (214, 314) and configured to receive a supplemental airflow that receives heat from at least one of the oil flow and the fuel flow, wherein the outer surface (217, 317) at least partly defines the supplemental airflow path; the heat exchanger system being characterized in that it comprises a control system coupled to one or more sensors that are configured to obtain a temperature of oil in the oil flow path and a temperature of fuel in the fuel flow path; and a valve member configured to be selectively and automatically actuated by an actuator coupled to the control system, the control system configured to selectively vary, via actuation of the actuator coupled to the valve member, a mass flow rate of an airflow through the supplemental airflow path using the obtained oil and fuel temperatures. a core structure (214, 314) at least partly defined by an elongate flow structure (216, 316), the elongate flow structure (216, 316) extending along an axis (218, 318) from a first end to a second end, wherein the elongate flow structure (216, 316) includes an outer surface (217, 317); an oil flow path (224, 324) that is internal to the elongate flow structure (216, 316) and that extends along the axis (218, 318) between the first end and the second end, the oil flow path (224, 324) configured to receive an oil flow; a fuel flow path (240, 340) that is internal to the elongate flow structure (216, 316) and that extends along the axis (218, 318) between the first end and the second end to be included in the core structure (214, 314), the fuel flow path (240, 340) configured to receive a fuel flow, the fuel flow path (240, 340) coupled to the oil flow path (224, 324) to allow the fuel flow to receive heat from the oil flow in the oil flow path (224, 324), the oil flow path (224, 324) and the fuel flow path (240, 340) being separated by a common wall within the elongate flow structure (216, 316), wherein in a cross section of the elongate flow structure (216, 316), the fuel flow path (240, 340) at least partly surrounds the oil flow path (224, 324) and wherein, in the cross section of the elongate flow structure (216, 316), the fuel flow path (240, 340) is disposed between the outer surface (217, 317) and the oil flow path (224, 324); a supplemental airflow path defined at least partly by the core structure (214, 314) and configured to receive a supplemental airflow that receives heat from at least one of the oil flow and the fuel flow, wherein the outer surface (217, 317) at least partly defines the supplemental airflow path; the heat exchanger system being characterized in that it comprises a control system coupled to one or more sensors that are configured to obtain a temperature of oil in the oil flow path and a temperature of fuel in the fuel flow path; and a valve member configured to be selectively and automatically actuated by an actuator coupled to the control system, the control system configured to selectively vary, via actuation of the actuator coupled to the valve member, a mass flow rate of an airflow through the supplemental airflow path using the obtained oil and fuel temperatures. a control system coupled to one or more sensors that are configured to obtain a temperature of oil in the oil flow path and a temperature of fuel in the fuel flow path; and a valve member configured to be selectively and automatically actuated by an actuator coupled to the control system, the control system configured to selectively vary, via actuation of the actuator coupled to the valve member, a mass flow rate of an airflow through the supplemental airflow path using the obtained oil and fuel temperatures.