Quora has an interesting thread on this. I should write on it. It’s a great example of people, including engineers, confidently saying all kinds of things that contradict each other. It’s also a good example of how people trust authority blindly without thinking of what the authorities’ financial incentives are. Lots of people say, “Follow the manufacturer”, but it’s clear the manufacturers have drastically changed their advice (even with regard to just synthetic oil) over the years, and different manufacturers offer contradictory advice for essentially the same kind of engine.