Insulating mineral oil is a petroleum fraction refined for high dielectric strength and low viscosity. Inside a transformer it does two jobs at once: it insulates, filling the gaps between windings and core, and it cools, carrying the heat by convection to the radiators.
Its condition governs that of the whole transformer. It ages by oxidation, forming acids and sludge that settle and obstruct cooling; it absorbs moisture, which lowers its dielectric strength drastically; and it breaks down locally under heat and arcing, generating the gases that DGA measures.
It is therefore put through a test programme: breakdown voltage to IEC 60156, water content, acidity, dissipation factor and dissolved gas analysis. When the values fall outside specification there are alternatives to replacement: filtering, vacuum dehydration and regeneration with adsorbent earths, which bring the oil back to specification without draining the tank.