The oil or fat you use for deep-frying should have a high smoke point — the temperature to which it can be heated without smoking. Butter and margarine have low smoke points, so they aren’t good for frying but work for light sauteing. The best oils for deep-frying and high temperatures are refined safflower and sunflower oils, peanut, safflower and soy oils. Refined almond, avocado and cottonseed oil are also great if you can find and afford them, and canola oil is usually not a problem either.
Remove food particles from used deep-frying oil by straining it through a coffee filter, or a sieve or funnel lined with a double layer of cheesecloth. Cover, tightly seal and refrigerate strained oil; it can then be used one more time.
The temperature of the fat is all-important if the fat isn’t hot enough, food will absorb fat and be greasy, oils that can't take the heat will get too hot, and burn. The normal temperature range for frying is 325°F to 375°F, however, it'd quite likely that higher temperatures of 375°F to 400°F also are used. Most foods cook rapidly in the 325°F to 375°F range and develop a golden color, crisp texture and good flavor. High-temperature frying leads to thinner crusts and less oil absorption. Foods fried in this normal temperature range absorb 8 to 25 percent oil. Frying time is longer at lower temperatures. Frying at lower temperatures results in lighter color, less flavor development and increased oil absorption.
P.S.
Smoke point is the temperature to which an oil can be heated before it smokes and discolors—indications of decomposition. If you are cooking with oil and it begins to smoke, you have reached its smoke point. At the smoke point, the oil begins to emit unpleasant odors and impart unsavory flavors to your meal. Watch out for the smoke point signs as it means you are getting close to the flash point, which is when the oil can erupt into flames. Knowing the smoke point warn you about the flash point and fire points. At the flash point, there are tiny wisps of flame; at the fire point a fire is blazing. The best oils for cooking and frying are those that have a high smoking point – that is, they can be heated to high temperatures before burning.
A number of factors will decrease the smoke point of any fat:
Combination of vegetable oils in products
Presence of foreign properties (batter)
Temperature to which oil is heated
Presence of salt
Number of times oil is used
Length of time oil is heated
Storage of oil (exposure to oxygen, light, temperature)