One of my lube system vendors told me that he is often repairing their oil cooling systems in the summer and it's a struggle to keep the gearboxes warm enough in the winter.

There aren't very many oil viscosities that can handle such a wide operating temperature range. On hot days the gears and bearings are not oiled well enough, and on cold days they are oiled too much which tends to blow out seals and add resistance which reduces the gearbox and wind turbine efficiency.