We used to use this method every day at the dealers and shops I worked at 40 years ago.
Fuel quality varied wildly between brands both name brands and off brands.
We got tons of complaints about pinging after a tune up. So we'd ask the customer to fill the tank with his favorite flavor if gas from his favorite station (many had water issues with their gas). Then we'd adjust the timing for the gas they were burning.
There was no other way.
The good thing was customers were loyal.nto a certain brand or station. They knew about cheap gas and tried to watch what they bought... No federal guidelines back then.
