How oftern should a deep fat fryer Oil be change

It depends on how much you use it. If you use every day all day them change oil once a week and cleaned at the same time. If used rarley then change oil when food tasted not as good as it shood.

I've got a double Imperial 40 lb fryer. I get one pilot lit, go to light the other one and the other one goes out. Also, I sometimes get both lit, turn the thermostat on for one of them and they both go out. Then sometimes have problems getting both of them to light period; the produce a flame but when I release the pilot light button, they go out!