A system for maintaining oil inside a gearbox of a wind turbine at least at a minimum level when an inclination of the gearbox causes oil to drain from the gearbox by gravity, the system comprising: the gearbox; an external oil tank for oil destined for the gearbox; and an intermediate tank, comprising: some oil inlets (8.1, 8.2) and oil outlets (9.1, 9.2) for connection between at least one multiplication stage (6.1) of the gearbox of the wind turbine and an external tank of oil for the gearbox, as many oil inlets (8.1, 8.2) as necessary to join all the oil outlets from the gearbox stages, the inlets (8.1, 8.2) and outlets (9.1, 9.2) are configured so that both the inlets (8.1, 8.2) as well as the outlet (9.1) to the external oil tank are located above the maximum oil level in the gearbox, while the outlet (9.2) to the last stage (6.1) in the gearbox is located just below the minimum oil level that could be present inside the gearbox, an upper outlet (9.1) for oil towards the external oil tank located above the maximum oil level on the intermediate tank, such that when the oil level exceeds the maximum oil level of the intermediate tank, the oil drains towards the external oil tank through the upper outlet (9.1) on the intermediate tank, and a lower outlet (9.2) for oil towards at least one multiplication stage (6.1) of the gearbox located under the minimum oil level on the intermediate tank, such that when the oil level is below the maximum level on the intermediate tank, the oil drains towards at least one multiplication stage (6.1) of the gearbox through the lower outlet (9.2) on the intermediate tank. the gearbox; an external oil tank for oil destined for the gearbox; and an intermediate tank, comprising: some oil inlets (8.1, 8.2) and oil outlets (9.1, 9.2) for connection between at least one multiplication stage (6.1) of the gearbox of the wind turbine and an external tank of oil for the gearbox, as many oil inlets (8.1, 8.2) as necessary to join all the oil outlets from the gearbox stages, the inlets (8.1, 8.2) and outlets (9.1, 9.2) are configured so that both the inlets (8.1, 8.2) as well as the outlet (9.1) to the external oil tank are located above the maximum oil level in the gearbox, while the outlet (9.2) to the last stage (6.1) in the gearbox is located just below the minimum oil level that could be present inside the gearbox, an upper outlet (9.1) for oil towards the external oil tank located above the maximum oil level on the intermediate tank, such that when the oil level exceeds the maximum oil level of the intermediate tank, the oil drains towards the external oil tank through the upper outlet (9.1) on the intermediate tank, and a lower outlet (9.2) for oil towards at least one multiplication stage (6.1) of the gearbox located under the minimum oil level on the intermediate tank, such that when the oil level is below the maximum level on the intermediate tank, the oil drains towards at least one multiplication stage (6.1) of the gearbox through the lower outlet (9.2) on the intermediate tank. some oil inlets (8.1, 8.2) and oil outlets (9.1, 9.2) for connection between at least one multiplication stage (6.1) of the gearbox of the wind turbine and an external tank of oil for the gearbox, as many oil inlets (8.1, 8.2) as necessary to join all the oil outlets from the gearbox stages, the inlets (8.1, 8.2) and outlets (9.1, 9.2) are configured so that both the inlets (8.1, 8.2) as well as the outlet (9.1) to the external oil tank are located above the maximum oil level in the gearbox, while the outlet (9.2) to the last stage (6.1) in the gearbox is located just below the minimum oil level that could be present inside the gearbox, an upper outlet (9.1) for oil towards the external oil tank located above the maximum oil level on the intermediate tank, such that when the oil level exceeds the maximum oil level of the intermediate tank, the oil drains towards the external oil tank through the upper outlet (9.1) on the intermediate tank, and a lower outlet (9.2) for oil towards at least one multiplication stage (6.1) of the gearbox located under the minimum oil level on the intermediate tank, such that when the oil level is below the maximum level on the intermediate tank, the oil drains towards at least one multiplication stage (6.1) of the gearbox through the lower outlet (9.2) on the intermediate tank.