Just FYI, several years ago I conducted a fatal fire investigation, which involved a deep fryer (electrical appliance).

Cooking oil contained into our deep fryer boiled and eventually degrated to auto ignition which resulted in kitchen fire that consumed part of the upper cupboard. The fire self-extinguished.

In order to conduct auto- ignition tests we boiled various type of cooking oils including both fresh and recycled oils.

We knew that our fire victim was known to often use his deep fryer. Like many other people do, we learned the victim recycled his cooking oil. Therefore, since it was not possible to quantify regular use, identification of the exact type of cooking oil involved and or state of cooking oil (recycled vs fresh oil) at the time of the fire, we conducted various tests with both fresh and recycled cooking oils (various type).

In order to complete our tests, I obtained a quantity of oil from restaurants and this included lard in addition to fresh oil. This was simply aimed at gathering data on possible variables.

I don't have access to our tests, however, to the best of my recollection we achieved auto-ignition in all metal pots and if I am not mistaken with minor variables with time of ignition.

Our work was aimed at testing the appliance under various conditions and comparison of state of cooking oil after auto-ignition was part of our analysis.

I hope this helps.