What is the typical chemical composition of transformer oil?

Mineral transformer oil is typically a highly refined mixture of hydrocarbons, mainly paraffinic and naphthenic components, with very low sulfur and aromatics compared to raw petroleum fractions. Inhibited oils include an antioxidant additive package to slow oxidation and sludge formation. Ester fluids differ by being based on fatty acid esters rather than petroleum hydrocarbons. Exact composition varies by supplier, specification, and whether the oil is inhibited, passivated, or designed for low temperature performance.