A manufacturer's manual or food product information on the package is the best guide for suggested cooking temperature. It also is important to periodically check the shortening temperature as it will drop significantly, while food is cooking. Some new fryers have a temperature display to allow a visual check of the shortening temperature. Controlling the shortening temperature is important for both food quality and safety. The thermostats are critical.
Fryers have two thermostats.
One thermostat is used to determine temperature of the shortening for food quality. The thermometer portion of the thermostat measures changes in shortening temperature and the power control part of the thermostat turns the heat source on and off to keep the desired temperature. Excessive overshoot, beyond the set temperature, can reduce shortening life. This can be minimized with an electronic thermostat. The second thermostat is a safety device called a high limit thermostat.