Ummm, that turbo diesel truck example is straight from Bitog. Take it up with them.
The Mustang's owner manual says what it says. It doesn't say use whatever you want and we'll cover your stupidity.
My own manual says use mfg approved oils. It doesn't say use whatever you want and we'll cover your stupidity.
I've seen, first hand, warranty claims denied for owner idiocy.
I'd rather not risk a $50,000 dollar car using non approved stuff during the warranty period, then spend thousands more on lawyers while it winds its way through a court in case of failure.
While the odds are that a failure would not occur, it is my car and my money. I don't want to get into a pissing match between Oil maker XYZ and BMW, for example.
Much easier to tell BMW "I used your oil, your interval, you changed it, and my engine died."
Much easier than telling them I used XYZ, then I get to be the ping pong ball while they paddle me back and forth between them.
Not much point in a warranty if you're not going to follow it, is there. Or in anyone offering one if it can be ignored.