They were built to run on it and something dreadful happens if they’re deprived.
Untrue. Use only the fuel recommended. Putting premium fuel in a car that requires only regular unleaded simply costs you more, and may actually reduce your mileage. The octane is higher, and the fuel will require higher temps/pressures to combust. In order to do this, the engine's computer will adjust the timing of the ignition and that may actually work against you in a car designed for regular alone.
Unless they actually call for premium in the manual, don't use it.
I haven’t seen a car in a long time that runs on plain unleaded.
[but I’d never buy a *new* car so who knows what they need, nowadays]
I think my ‘73 Saab 96 will run on kerosene and lard but I’m not sure....;D