For a short period of time I worked at a local filling station, long before the days of self service pumps & some drivers were quite abrasive when demanding their GS stamps. I got my own back on them, if it was raining, by making sure the stamps got damp on the journey back from the office to their car
I think Kermit is right and it was to do with the additives to reduce knock. We certainly put 2* in our mini because 4* was more expensive. 2* went in the mowers too.
Yea lead in 4star ,stopped engine knock,pinkering ,ran cooler and lubricated valves so they didn't burn away ,..Remember seeing a program about the grass n vegetation at the side of motorways was toxic with lead ,before the introduction of unleaded fuel never mind what wee were all breathing in traffic jams..