Sounds like one of the many ustaad tales. Conspiracy theorists love them and most of our awaam is seasoned conspiracy theorist.
You should ask him what Used Oil Analysis has he done (or even virgin oil analysis) and in what high tech lab that other auto enthusiasts don’t know about. Plus share the specific numbers like HSTS, TBN etc.
Otherwise it’s just a mirch masala story.
No harm in buying from dealership, their oil also meets all specs like API SN/SP, grade etc. only reason I don’t buy from dealership is because they don’t sell the oil that I prefer and even if they were selling, usually they sell at a price higher than the market rate.