A method of flushing a cooling circuit for gear oil of a gearbox in a wind turbine, the cooling circuit comprising a gear oil pump and a heat exchanger, the cooling circuit being connected to the gearbox and configured for circulating gear oil from the gearbox through the cooling circuit by the gear oil pump during standard operation of the wind turbine when producing electricity; the method comprising connecting a pressure release valve to the cooling circuit downstream of the heat exchanger between the heat exchanger and the gear box, the pressure release valve being configured for closing for flow of flushing liquid, for example flushing oil, from the cooling circuit into the gearbox and for opening for flow of flushing liquid when the pressure in the cooling circuit rises above a predetermined pressure limit; providing a flushing circuit upstream of the heat exchanger and different from the cooling circuit, the flushing circuit comprising an upstream end and a downstream end and therein between a flushing filter and an air supply (16, 17) downstream of the flushing filter; by a first pipe connecting the upstream end of the flushing circuit to the gearbox or to the cooling circuit upstream of the exchanger for receiving flushing liquid, for example flushing oil, from the gearbox into the flushing circuit through the first pipe and the upstream end; by a second pipe connecting a downstream end of the flushing circuit to the cooling circuit at a downstream connection location upstream of the heat exchanger; pumping flushing liquid, for example flushing oil or gear oil, from the gearbox through first pipe, the upstream end and the flushing filter, pass the air supply (16, 17) and through the heat exchanger, the downstream end and the second pipe, and adding air from the air supply (16, 17) to the flushing liquid in the flushing circuit while the flushing liquid is passing the air supply (16, 17); while the pressure release valve is closed, building up pressure to a predetermined level in the flushing circuit and in the cooling circuit and, upon reaching this predetermined pressure limit, opening the pressure release valve downstream of the heat exchanger and causing a turbulent flow in the cooling circuit during a forceful release of pressure from the cooling circuit through the pressure release valve. connecting a pressure release valve to the cooling circuit downstream of the heat exchanger between the heat exchanger and the gear box, the pressure release valve being configured for closing for flow of flushing liquid, for example flushing oil, from the cooling circuit into the gearbox and for opening for flow of flushing liquid when the pressure in the cooling circuit rises above a predetermined pressure limit; providing a flushing circuit upstream of the heat exchanger and different from the cooling circuit, the flushing circuit comprising an upstream end and a downstream end and therein between a flushing filter and an air supply (16, 17) downstream of the flushing filter; by a first pipe connecting the upstream end of the flushing circuit to the gearbox or to the cooling circuit upstream of the exchanger for receiving flushing liquid, for example flushing oil, from the gearbox into the flushing circuit through the first pipe and the upstream end; by a second pipe connecting a downstream end of the flushing circuit to the cooling circuit at a downstream connection location upstream of the heat exchanger; pumping flushing liquid, for example flushing oil or gear oil, from the gearbox through first pipe, the upstream end and the flushing filter, pass the air supply (16, 17) and through the heat exchanger, the downstream end and the second pipe, and adding air from the air supply (16, 17) to the flushing liquid in the flushing circuit while the flushing liquid is passing the air supply (16, 17); while the pressure release valve is closed, building up pressure to a predetermined level in the flushing circuit and in the cooling circuit and, upon reaching this predetermined pressure limit, opening the pressure release valve downstream of the heat exchanger and causing a turbulent flow in the cooling circuit during a forceful release of pressure from the cooling circuit through the pressure release valve.