The generation of electricity by wind power is becoming very important due to the concerns about the effects of global warming [5,6]. To make wind energy competitive with other power plantsthe near future, enhancements on availability, reliability and lifetime of the wind turbines will be required [7–10]. For this particular case, the wind turbine gearbox problems can start with the oil that should be treated with caution as a mechanical component. According to DIN recommendations the best viscosity and anti-scuffing properties are reached for oil operating temperatures above 80 1C. In wind turbine gearboxes where operating temperatures sometimes do not exceed 60 1C the anti-scuffing class tends to drop, resulting in a worst start-up behaviour and higher debris level generated [11]. A major problem with this kind
of application is to assure an effective lubricant film and the oil must be synthetic if the oil sump temperature is higher than 80 1C [12,13].