How do I open a padlock after it has frozen?
Get some lock deicer/lubricant. It's sold specifically for this and contains graphite and methanol. Shake well before applying. The methanol removes water and oil from the lock mechanism and leaves behind graphite well flooded through every nook and cranny in the device. You use graphite because it's a dry lubricant and unlike oil, it doesn't attract dirt or gum up like petroleum lubricants.