I've always been someone who changes their engine oil (and filter etc) regularly. In the days of mineral oils, every 5/6k miles or 6mths, with synthetic oils, 10/12k / 12mths, assuming the vehicle wasn't spec'd for less.
I've a friend you swears he never changes his oil and has, or had (I've not seen him for awhile) an ancient lorry (an HGV) which he insists he'd never changed the oil in, just topped it up. Admittedly, it was only used for perhaps 6 or 7 mths of the year and even then covered only a few 1000 miles, if that. (He was a showman with a ride who traveled and set it up at various events.)
I've looked at several videos on YouTube covering DIY recycling of oil, essentially filtering it to remove the 'larger' bits, then leaving the rest to settle, after which (supposedly), you are left with 'clean', almost clear as original, oil.
While I am prepared to accept this may remove the solids, any other dissolved 'gunge' will still be they, plus if the oil has changed in structure/natured, it isn't as it was.
I'm not remotely thinking of reusing engine oil, the hassle isn't worth it, but I am curious.
Some further reading has shown that oil is recycled on a commercial scale by being broken down (fractioning) in a way not unlike how crude oil is processed. However, this is a complex process, with quality checks etc.