A gearbox of a drive train of a wind turbine, the gearbox having an oil inlet and oil outlet and a lubrication system, the lubrication system comprising, an oil reservoir having an outlet, a supply valve, a drain valve (V1, V2), the drain valve (V1, V2) coupled to the oil outlet of the gear box configured to close in the off-grid state of the wind turbine, wherein the oil reservoir is coupled to the supply valve and the supply valve is coupled to the inlet of the gearbox, and the supply valve is configured to open in an off grid state of the wind turbine, wherein the gearbox comprises a siphon (500, 501) configured to adjust an internal oil level (122, 126) in the gearbox in the off-grid state of the wind turbine, the siphon having a first end, a second end and an elevated part between said first end and said second end, wherein the oil outlet of the gearbox is coupled to the first end of the siphon, and wherein the second end of the siphon (500, 501) is coupled to a second side (332, 334) of the drain valve (V1, V2) and the geodetic height of the elevated part corresponds to the internal oil level (122, 126) during the off- grid state of the wind turbine, wherein the gearbox is a two stage gearbox and the first stage has a first internal oil level and the second stage has a second internal oil level in the off-grid state of the wind turbine, wherein the first stage of the gearbox has a first oil outlet and the second stage of the gearbox has a second oil outlet, wherein the gearbox further comprises a second siphon having a first end and a second end and wherein the first end of the second siphon is coupled to the second oil outlet of the second stage of the gearbox and the second end of the second siphon is coupled to the first oil outlet of the first stage of the gearbox. an oil reservoir having an outlet, a supply valve, a drain valve (V1, V2), the drain valve (V1, V2) coupled to the oil outlet of the gear box configured to close in the off-grid state of the wind turbine, wherein the oil reservoir is coupled to the supply valve and the supply valve is coupled to the inlet of the gearbox, and the supply valve is configured to open in an off grid state of the wind turbine, wherein the gearbox comprises a siphon (500, 501) configured to adjust an internal oil level (122, 126) in the gearbox in the off-grid state of the wind turbine, the siphon having a first end, a second end and an elevated part between said first end and said second end, wherein the oil outlet of the gearbox is coupled to the first end of the siphon, and wherein the second end of the siphon (500, 501) is coupled to a second side (332, 334) of the drain valve (V1, V2) and the geodetic height of the elevated part corresponds to the internal oil level (122, 126) during the off- grid state of the wind turbine, wherein the gearbox is a two stage gearbox and the first stage has a first internal oil level and the second stage has a second internal oil level in the off-grid state of the wind turbine, wherein the first stage of the gearbox has a first oil outlet and the second stage of the gearbox has a second oil outlet, wherein the gearbox further comprises a second siphon having a first end and a second end and wherein the first end of the second siphon is coupled to the second oil outlet of the second stage of the gearbox and the second end of the second siphon is coupled to the first oil outlet of the first stage of the gearbox.