Mineral oil
Mineral oil is generally effective as a transformer oil, but it has some disadvantages, one of which is its relatively low flashpoint versus some alternatives. If a transformer leaks mineral oil, it can potentially start a fire. Fire codes often require that transformers inside buildings use a less flammable liquid, or use dry-type transformers with no liquids at all. Mineral oil is also an environmental contaminant, and its insulating properties are rapidly degraded by even small amounts of water. Transformers are well equipped to keep water outside the oil for this reason.