Transformer oil in oil-immersed transformers is used for insulation, cooling and arc extinction. Then what oil is in the transformer case?

Transformer oil is a kind of mineral oil obtained by distillation and refining in natural petroleum. Transformer lubricating oil is a mixture of liquid natural hydrocarbons with pure stability, low viscosity, good insulation and good cooling ability after refining of lubricating oil fractions in oil by acid and alkali.

Transformer oil is commonly known as square shed oil, light yellow transparent liquid.