Thin Film Oxidation - All fluids oxidise, but it takes years.
In non-free breathing transformer applications (transformers where ambient air and oil are not in continuous, direct contact for extended time periods), oxidation is not an issue as fluid exposure to the atmosphere is minimal.
Even in a free-breathing transformer, or with intermittent exposure (as experienced in a transformer that leaks), oxidation takes years.
Each fluid type oxidizes differently and the byproducts of oxidation may affect the performance of a transformer. As such, varied laboratory oxidation stability tests have been developed to be used as control tests for evaluating the relative oxidation stability of dielectric fluids in a laboratory environment; but they are not intended to simulate operational performance within a transformer.