People, I've got an application with two identical gearboxes. Large vertical shaft plantetary gear sets, running around 400RPM and 2,000 hp driving through them. Both are operating under the same conditions at the same time. One operates normally, and the other's oil foams and looks like expanding foam. Checked for moisture (cooler leaks), and there is no water present. Changed the oil (twice), and it still foams. Any ideas ?