An oil system for a turboprop gas turbine engine adapted to drive a propeller of an aircraft, the propeller having a plurality of variable pitch propeller blades, the oil system comprising: an engine oil return system; and a propeller blade angle control circuit, comprising: a propeller control unit controlling a supply of oil output therefrom adapted to be used to modify a pitch angle of the propeller blades; a fixed-displacement pump located upstream of the propeller control unit and providing the supply of oil from the engine oil return system to the propeller control unit; and an oil cooling line extending between an outlet of the pump and the engine oil return system to define an oil leakage path leading to the engine oil return system for cooling the oil, characterised in that: the propeller blade angle control unit comprises a flow regulator between the pump and the propeller control unit, the flow regulator being fluidly connected to the oil cooling line and operable between an open position and a closed position, wherein in the open position the flow regulator directs oil through the oil cooling line along the oil leakage path to the engine oil return system for cooling the oil and in the closed position the flow regulator blocks the oil cooling line and directs oil toward the propeller control unit, in use, to modify the pitch angle of the propeller blades; and wherein the flow regulator includes a pressure regulator valve (31; 131; 231) having a housing (32; 132) with an inlet aperture in fluid communication with the outlet of the pump and an outlet aperture (34; 134; 234) in fluid communication with the oil leakage path leading to the engine oil return system, the pressure regulator valve (31; 131; 231) having a spool (36; 136; 236) disposed within the housing (32; 132) and displaceable relative to the housing (32; 132) to block and expose the inlet aperture. an engine oil return system; and a propeller blade angle control circuit, comprising: a propeller control unit controlling a supply of oil output therefrom adapted to be used to modify a pitch angle of the propeller blades; a fixed-displacement pump located upstream of the propeller control unit and providing the supply of oil from the engine oil return system to the propeller control unit; and an oil cooling line extending between an outlet of the pump and the engine oil return system to define an oil leakage path leading to the engine oil return system for cooling the oil, a propeller control unit controlling a supply of oil output therefrom adapted to be used to modify a pitch angle of the propeller blades; a fixed-displacement pump located upstream of the propeller control unit and providing the supply of oil from the engine oil return system to the propeller control unit; and an oil cooling line extending between an outlet of the pump and the engine oil return system to define an oil leakage path leading to the engine oil return system for cooling the oil, the propeller blade angle control unit comprises a flow regulator between the pump and the propeller control unit, the flow regulator being fluidly connected to the oil cooling line and operable between an open position and a closed position, wherein in the open position the flow regulator directs oil through the oil cooling line along the oil leakage path to the engine oil return system for cooling the oil and in the closed position the flow regulator blocks the oil cooling line and directs oil toward the propeller control unit, in use, to modify the pitch angle of the propeller blades; and wherein the flow regulator includes a pressure regulator valve (31; 131; 231) having a housing (32; 132) with an inlet aperture in fluid communication with the outlet of the pump and an outlet aperture (34; 134; 234) in fluid communication with the oil leakage path leading to the engine oil return system, the pressure regulator valve (31; 131; 231) having a spool (36; 136; 236) disposed within the housing (32; 132) and displaceable relative to the housing (32; 132) to block and expose the inlet aperture.