TAN increase in hydraulic unit

most of the time increasing TAN may be result of over heating of oil and may be oil gets in contact with air.

Hey,

Whats is the oil working temperature?

Have you notice any stainless stell color changing or something like that?

The oils always degrade but if it a stable one takes longer and if it a metastable one will taker less.
For example hydrocarbons are relatively stable, especially as higher the structure as it happens for oils. So its degradation is so slow that even temperature cannot raise the kinetics of the reaction enough to be extremely significant.
According to the Arrhenius index, each 10ºC above 60ºC will double the speed of the degradation reaction. Oxygen degrades by forming peroxides which then breack forming smaller molecules (ketones, aldehides, etc.) which give smell to the oil. The presence of light interferes by activating the oxygen, so it is more reactive.