It's not that it's an "inferior gas". It's good fuel for normal cars. Not tuned performance cars. From what I read and have heard from 2 tuners they think it has something to do with the additives. Most tuners don't like it because it causes knock retard. Same thing you're experiencing. Look it up. You'll see tons of cases where people are having knock issues and using shell gas. They switch and it goes away.
There are very few things that can cause knock retard. But if you're commanding a certain degree of timing and the car is pulling it in my experience it can only be fuel or heat. It's not hot so that leaves fuel.
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