Insulating fluids can be categorised in many different ways. One of the most common classifications would be by identification of the constituent base oil: mineral oil, synthetic, or natural oils. Mineral oil is derived from crude oil, this type of oil is known for specific characteristics - chemical and electrical characteristics - that are highly reliant on the specific process of refining that is used during the manufacturing of the oil.
Natural esters are derived from a 100% renewable source, vegetable oils are superior to mineral oil in some instances and for some applications. They can be used for power transformers and are especially well suited for distribution transformers. These types of transformers can be retro-filled using natural or synthetic ester fluid if previously filled with mineral oil. The manufacturer should be contacted to confirm if the retro-fill is advisable for that specific design, as ester fluids might flow slower through the transformer, causing lower heat dissipation.