Been around heavy forestry and ag equipment all my life and been doing my own repairs for 40 years or so, including transmissions, engines, differentials and final drives. Not claiming to be an expert, but in my experience the only time that I have ever seen rust on the inside of a casing was on machines that have sat for long enough to let the oil coating dry off the casing and internal parts above the oil line. It can happen quite quickly in areas where there is significant temperature variation like here in Maine. A warm day and followed by a cold night causes condensation on any metal surface, including gear surfaces, wet brake disks, etc. I always start each of my tractors at least every two weeks in winter and take them for a ride in the snow. They get heated up nicely, I get to look around my place, the housings, bearings, and surfaces in the machine are recoated with oil, the battery gets recharged nicely, and my neighbors wonder what the **** I'm doing. All very worthwhile outcomes. My $0.02 worth.
