Patrickt 65 years ago my dad (who owned a garage and started my career as a gear head) told me that he sold the old used
oil back to a few
oil companies. He said they refined it and use it in installations that needed finer
oil. Never thought about it again. I also attended a class on rebuilding Solar Turbines which used Mobile 1 oil and ran at 20,000 RPM. Also didn't change oil, but sent samples to lab. They would tell us when to replace oil or any part that should be replaced (mostly bearings since not many moving parts).