I've always wondered about these high efficiency oil filters that truckers use, like those from Gulf Coast Filters (just a random company I Googled). If you look on their page, they have various examples of rigs that have gone hundreds of thousands, even a million miles with no oil changes or just a single one, for the million mile rig. (Obviously, the filter needs to be changed regularly)

And according to the GCF website, additives mostly break down in response to dealing with the various contaminants that build up in an engine. They claim that since their filter keeps the oil very, very clean, the additives last a very long time because there is little contamination for the additives to interact with.