Simple test if you really want to know the truth about mineral oil in cold weather:
Put some in the freezer and see what happens. Check it at various points as it cools over a few hours.
It goes thicker and thicker. I noticed this a while ago when I accidentally put some olive oil in the freezer. It seems to explain why Shimano's go kind of dead feeling in the cold.
As some others may have mentioned though, it only happens when its below zero