In hydraulic elevators the base oil typically does not wear out.  The additive package may be depleted and some of the oil may oxidize, but the bulk of the oil can be scrubbed.

Oil that oxidizes is no longer oil; it is chemically changed into oxidation byproducts which can be removed from the oil using depth filtration.  Due to the large volume of oil in hydraulic elevators, most of the oil is not oxidized because it does not encounter the heat zones in the pump, valve or elbows.