All fuel has a minimum of cleaners added. No where can you find what or how much is added to make them Top Tier. Around me Top Tier is not even mentioned and the gas prices are all in line with each other, Top tier or not.
Top tier fuel has 2-3x the amount of detergents required by EPA. Gasoline used to have this amount required by the EPA. In the 1995, to lower the price of gasoline for poor people that voted for the dress staining president, the fuel detergent level requirements dropped. When your democrats in gov't screw up, sometimes an organization takes over with better requirements. Saving the voter a couple pennies a gallon was more important than good running vehicle and the environmental consideration. Demo-hypocrisy at work.
Its not a fad. I remember when very few were on the list. And, I also remember when a few major brands claimed "as good as top tier" but didn't want to pay the licensing fee. Here is the current list. https://toptiergas.com/licensed-brands/
and a older video showing a couple toptier gas detergent residue levels compared to lesser brands(some that might've signed up since then as the list is ever changing): https://youtu.be/ystuUJWae0Y?t=155
Over the years, may stations have advertised 'top tier'. Its common now. Look at all the commercials, for every imaginable product from the 1970's to 1980's, still being sold, but rarely have tv commercials. TopTier just that common now, even when consumers have no common sense.
GDI intake/ports don't benefit as much from TopTier fuel. But, at least the injectors/pistons/combustion chambers.... will stay cleaner. Can't win them all.
Every brand of gas station can treat their fuel 'wherever/however they want', at the tank farm, at the tanker, or at the station. I don't know who does what. But, I do know, from working at a tank farm, that even with a row of trucks at the same refilling point, trucks can get different fuels by opening/closing different valves at the fuel depot. Can you not get 3 different octanes at the same gas pump? Well the tanker can get different brands/octanes/detergent levels at the refill point too!