The presence of both dissolved and free water in wind turbine gearbox lubricants is known to reduce bearing fatigue life significantly and also promotes the onset of early life failure by white etching crack (WEC) and white structure flaking (WSF).
Dissolved water below the saturation limit is still of importance, but many operators and maintenance engineers measure water content of gearbox oils in order to avoid reduced bearing lives. However, oils (and even the same oil with a different additive package) have different saturation levels of water, that increase with temperature.
The physico-chemical effect of water is not governed by absolute level of water in ppm but by relative humidity. Water content of oil is dominated by the relative humidity of the air space in the oil tank. Water can enter the tank by seal leakage, but its entry is principally through breathers or filler caps. Hence the use of desiccant breathers.
Whitten and Stelson have prepared a useful paper on this topic, accessed vis the link below. They measured saturation limits for a few common gearbox oils. They found that the oil takes several days to equilibrate with the air space in terms of relative humidity. They highlight the need to maintain an air space with low relative humidity.