What does oil in a transformer usually consist of?

Oil in a transformer usually consists of mineral oil, which is derived from refining crude oil. This mineral oil is often treated with additives to enhance its properties, such as oxidation stability and electrical insulation. Additionally, some transformers may use synthetic esters or natural esters as transformer oil alternatives, which provide better biodegradability and fire resistance. Overall, the composition of oil in a transformer is crucial for effective cooling and insulation.