What s that Oil for again It says the main reason for using oil in old large capacitors and transformers is because of the heat generated Funny I thought it was used mainly as an insulating medium and dielectric OK its for cooling as well But not mainly for cooling in capacitors as they should nt get very hot unless thers a lot of ripple current and a high ESR So I intend to correct this statement I ve posted a dispute tag in this area because I do nt believe in general that oil is used for capacitor cooling Quote from manufacturer literature Why are GAEP high voltage capacitors oil filled Highly insulating oil is used to suppress partial discharges corona increase the dielectric strength and increase the effective permittivity dielectric constant of the capacitor dielectric Oil filled film capacitors have high energy density in comparison with other types of capacitors used at high voltage such as ceramic capacitors Different types of oils are used with different types of capacitor dielectric and electrode designs