Not every fry oil or shortening is ideal for every purpose. Fry oils are typically categorized into light/medium-duty oils, heavy-duty oils, and liquid butter substitutes:
Light/medium-duty oils include traditional liquid vegetable oils such as canola, soy, sunflower and olive oil. These oils can be used for salad dressings, marinades, sautéing, stir-frying and deep-frying. When used at high heat, light/medium-duty oils break down more quickly than heavy-duty oils and fry oils that contain trans fat. If you deep-fry with these oils, you must change oil more often than you would for heavy-duty fry oils.
Soy and canola oils are often treated with an antioxidant (such as TBHQ). The antioxidant is usually paired with an anti-foaming agent (such as dimethylpolysiloxane) that further slows the oil’s breakdown. These additives make cooking oil more durable for restaurant use. Light/medium-duty oils with additives may hold up a little longer in the deep fryer, but they still require more frequent changing than heavy-duty oils.
You can extend the fry-life of a low-cost light/medium-duty oil by blending it with a small quantity of heavy-duty oil. See “Understanding Heavy-Duty Fry Oils” for more information.
Heavy-duty oils are good for extended deep-frying over longer periods. (Under the right conditions, certain heavy-duty oils can last a week or more.) These oils differ in flavor, cost and stability. They are available as clear oil or creamy, pourable shortening. Heavy-duty oils and shortenings with 0 grams of trans fat will perform like partially hydrogenated fry products.
Liquid-butter substitutes are used for griddle and pan frying. These are butter-flavored 0 grams trans fat shortenings and margarines (creamy and solid) that are much more stable than butter at high heat.