Personal experience....I keep my DR650 in the shed, and I'm on the east coast (Canada, ehh?). I took it for a last rip, got it good and hot, then changed the oil and rolled it into the shed. When the good weather started approaching, it was warm enough to get in the shed, pull the side cover off and replace the neutral sending unit screws with drilled allen bolts and safety wire them. One of the DR-isms is that the screws securing the sending unit are seated against the plastic housing and hot/cold cycles will allow them to loosen over time, dropping out and ending up jammed in between stuff inside the bottom end. Anyway...I pulled the side cover, and there was a large amount of water pooled anywhere it could lay on a flat surface, large beads of water on the bearings. Again, this was brand new oil, changed just before rolling it into the shed for its winter sleep.
In a below-freezing climate, you'll see cold days, warm days, and bright, sunny days. During those days, you'll often see frost coating the outside of the case...when it was way below freezing, then warmed up fast in the shed due to a temperature rise plus the sun beating on the uninsulated shingle roof. The inside of the shed would be suddenly warm and humid, but the metal of the engine would hold its cold for a long time, resulting in the moist, warm air in the shed condensing and freezing on the engine case. All I can figure, is that because it's a vented space, the warm, moist air is also drawn into the case and the same condensation/frost forms inside. Then when the engine warms up, it melts and gravity causes it to drip down. Each time there would be a cold night followed by a warmish sunny day, the process repeats. And I'd assume that the water that dripped down would freeze the next night, with the oil covering it, then remain frozen due to the metal and oil covering it, then it would be joined by the new water from that day, and so on...
In a heated garage, this wouldn't be an issue, but in a non-insulated shed, I'd imagine it would go through dozens of swings between well below freezing to well above.
After seeing the amount of water that collected in the case of the DR, I will now be starting the bikes in the spring, taking them for a run to get up to operating temperature and to mix the water and oil together, then changing it out while the water is still suspended.
If you click the attached pic and zoom in a ways, you'll see the amount of water that the DR collected over the winter. Again, that oil was dumped, refilled, then the bike was rolled into the shed. Zero running time on it. Also, I have no idea how much water was really laying in the bottom of the case...to do the NSU screws, you can lean the bike way over to the left side and all the oil will run through that oblong hole at the bottom and stay on the low side, so you can pull the cover without having to drain the oil. Note the big drops on the beige plastic gear, and the actual pools of water at the bottom of the case. If there's that much water left with the bike laid over 45*, there must be a LOT more over in the other side of the case.
I don't want to start a big thing on leaving dirty oil in the case over the winter and acids and all that...but, be aware, if the conditions are right (or wrong), your fresh clean oil might be severely water-contaminated come spring. And, if you just roll the bike out and check the level, all will look fine...the water will all still be laying on the bottom. It won't be until you start riding that it will mix with the oil. Just wanted to share my personal experience.