A pump system for supplying lubricant to components of a wind turbine comprises: a pump for pumping lubricant through a fluid circuit of the wind turbine; a drive means for driving the pump; and, a gearbox arrangement arranged to couple the drive means to the pump. The gearbox arrangement comprises a rotatable input shaft configured to be driven by the drive means and a rotatable output shaft configured to drive the pump. The input shaft is rotatable in a first direction of rotation and a second direction of rotation when driven by the drive means. The output shaft is rotatable in the first direction of rotation, and the speed of rotation of the output shaft is determined by an operational mode of the gearbox arrangement. In a first mode of operation, when the input shaft rotates in the first direction of rotation at a first speed of rotation, the gearbox arrangement is configured to drive the output shaft to rotate also in the first direction of rotation at the first speed of rotation. In a second mode of operation, when the input shaft rotates in the second direction of rotation at the first speed of rotation, the gearbox arrangement is configured to drive the output shaft to rotate in the first direction of rotation at a second speed of rotation.