Buckstones:
For what it is worth I see lots of taxis (cop cars too) filling up at the local Shell, there are several supermarkets not far away a couple of pence cheaper, so presume they find it worth the extra. I run my petrol car on Shell, at MOT time the emissions are usually spot on, its on 59,000 miles.
Possibly because they have an account there and use fuel cards hence why they only use the place,and your car or any car shouldn’t be having emission issues at 59K regardless of what fuel you/they fill up with…Placeslike shell & BP just like you to think that their “premium” fuel will help.