Oil that is frequently reheated in a deep-fryer is likely to become rancid in time. Its vitamin content also reduces, the flavor may become unpleasant, and it is possible for harmful substances to be produced if the oil is overheated. Discard any frying oil that smells unpleasant, produces too much foam when heated, creates smoke easily and has become dark in color.
In general, you should reheat oil two or three times only before discarding it. Each time it is used, the oil should be allowed to cool completely, then be filtered through a fine sieve. Once you have cooked a specific type of food in a batch of oil, re-use the oil for that food only.
** Asian Recipes