The fryer's cooking medium is hot oil, also known as shortening, frying compound or fat. The quality of the final food product largely depends on the quality of this oil. Flavors developed in the oil transfer to the foods being cooked. Also, oil is expensive, ranging from 30 to 75 cents per pound. Since a single fryer's oil capacity can range from 28 to 110 pounds, the cost for replacing used oil can be significant.

Food particles eventually degrade the oil. These particles continue to cook long after the final food product is removed from the fryer. They can eventually burn, leaving a bitter taste in the oil. Most fryers have a cold zone located at the bottom of the fryer where food particles can collect to minimize this problem. The temperature in this zone is lower than the cooking zone so food particles don't continue cooking. The fryer operator should frequently filter the oil to remove excess food particles and prolong the life of the oil.

Excessive temperature can also destroy cooking oil. If the fryer's temperature exceeds 400° F, the oil will begin to break down and develop a bad taste. Thermostat overrides and hot spots along burner tubes in gas fryers are frequent culprits.

Cooking temperature also greatly affects the quality of the final food product. Cooking at a high temperature tends to cook food faster on the outside and may overcook the outside while leaving the interior portion partially uncooked. On the other hand, cooking too slowly allows the food to absorb more oil making it soggy and adding to food preparation costs.