A detergent oil has additives in it to keep any condensed/accumulated water in suspension. In a automobile engine the oil get hot enough for the condensed water to evaporate over time.
In a gearbox, the oil does not get hot enough and the condensed water will build up over time. The oil will turn to an ugly mayonaise given enough time between changes.
With non-detergent oils, the water will drop out and can be drained away by just letting a little oil out of the bottom of the gearbox.