Electrostatic filters are great things but they operate very slowly and lose effectiveness as moisture content rises. There are some reports that electrostatic filtration will remove bi-products of oxidation and can filter to well beloew 0.5 microns. There are some plastic injection mouldings machines that actually use electrostatic filtration (I think, but don't quote me, Kawaguchi are one). As for the additives, in my opinion, they can cause more problems then if they were absent. But if you know what you are doing, sweetening the oil with new additives can be effective for life extension, although I would advise extreme caution with this approach.
Oils theoretically could last forever in the right conditions. But in my opinion, there are too many factors such as oxidation that cause a chemical breakdown of the oil. So you can keep removing the contamination but eventually, there will be none left.