A long time ago I worked in a professional kitchen and the clean french fry oil would never give us issues; however, the oil used for breaded fish and chicken needed to be cleaned regularly as the burned food bits caused the oil to "break", a term used to mean smoking and very dark oil.
The french fry oil only had to be changed 1/3 as often as the breaded use oil.
I have successfully brought "dirty" corn oil to auto ignition several times, a couple time on purpose.
I would think the impurities are the wild card.
Timothy G. Lomprey, B.S.
CFI, ECT, CFEI, CVFI
Wildland O & C
Lomprey Investigations, LLC
Las Vegas, Nevada