Why should transformer oil not be used in diesel engines?

Transformer oil is not a certified fuel. Its viscosity, flash point, additives, and contamination profile are not controlled for combustion, emissions, or engine wear. Burning it can generate toxic smoke and deposits, damage injectors, and violate environmental regulations. The risk is far higher if the oil is aged or contaminated, and catastrophic if PCBs are present because combustion can create extremely hazardous byproducts. Proper disposal or recycling must follow waste and hazardous materials rules, not fuel use.