“Can you filter out oxidation, and if so, what is the best method? I have heard that oxidation is a permanent chemical change and cannot be filtered out.”
Figuratively, oxidation can be filtered. That is, it can be slowed or reduced through proactive maintenance practices (cleaner, dryer, cooler, etc.).
In reality, oxidation cannot be filtered because it is a chemical aging process driven by catalysts such as high temperature, water, air, metals (in the form of wear debris and contamination) and other contaminants such as fuels and process chemicals. Hence, minimizing the ingress of these will reduce or significantly slow the oxidation rate of the oil, resulting in longer lubricant life.
However, byproducts of oxidation such as acids and fine polar insolubles can be removed by the use of advanced separation technologies such as electrostatic separators, ion-exchange resins and activated alumina. Additionally, dense absorbent depth-media (compressed cellulose, etc.), such as commonly used on bypass and off-line filters, can be effective at removing sludge and oxide insolubles. Because oxidation is auto-catalytic, the removal of oxides can help slow further oxidation.
Once these byproducts have been scavenged from the oil, the anti-oxidants will have likely been depleted. In many cases, the anti-oxidant can be reconstructed on the guidance of your lubricant supplier. A bleed and feed is sometimes recommended to refresh additives. Of course, once the oxidation process reaches a certain advanced stage, the oil’s properties may be too severely impaired to continue in service.