You can leave the oil overnight in your fry pot, but make sure this does not exceed more than a few days. To keep the oil at a good quality, filter it once it cools to remove any food remains. These will decay if left for too long and spoil your oil. If you leave it overnight, cover the top of the fry pot until when you next use the oil. However, it is better to empty your fryer and place the oil somewhere cool and dark so that the breakdown process is slowed.