Why is transformer oil unsafe and unsuitable for cooking or food use?

Transformer oil is an industrial insulating liquid, not a food grade product. It can contain additives, oxidation byproducts, dissolved metals, and contaminants from service. Legacy oils may be contaminated with PCBs or other hazardous substances, and even non PCB oils are not evaluated for ingestion safety. Heating transformer oil can release irritating fumes and decomposition products, and spills create environmental hazards. Use for cooking or food contact is unsafe and likely illegal in many places. Only approved edible oils should be used for food preparation.