As a former gasoline retailer let me say a few words about fuel formulation...

The difference between "regular" and "premium" unleaded fuels is only in
the additive package.  In the typical 20k gallon tank truck load the
additive package is typically less than 6 fluid ounces.

Arco fuel is it's own breed.  Usually less expensive for two reasons.
1.) Cash only.  Arco chooses not to deal with credit cards.  That saves
the retailer 3% off the top. 2.) Arco fuel (at least in the PNW (and CA
IIRC)) is about 10% ethanol.  This allows a lesser grade of gasoline to
meet the octane requirements of a higher grade. At the cost of less
specific energy.

As a former pipeline land surveyor:
Fuel is piped from the refineries to the distribution centers.  It all
goes in one (typically) 26" pipe. The system on the west coast is AFAIK
almost entirely owned by Chevron. It all comes out of one system. The
difference between Shell, BP, Mobil, Texaco, Chevron etc. is only the
few ounces of modifiers added. Gasoline is gasoline.

An aside:  My wholesaler delivers to all major brand stations in this
area (Texaco, Chevron, BP, Conoco).  Same truck, same fuel from the same
tank farm. The difference between them is the teacup full of exotic brew
that he adds to his tanker before leaving the tank farm. Different
brands = different majik chemicals. (BTW, he says respirator and
neoprene glove and apron are a must if one doesn't want to die young.
Strong stuff.)

Another aside:  Last time I purchased Arco fuel, my fuel mileage dropped
to 15.5 from the usual 18.5 mpg.  And Bleu ('77 2.0) did not like it at
all. Ran hot and rough.  Fuel was purchased from what is probably the
highest volume retailer in the state so 'freshness' was not an issue.
Filled again with the usual brand and everything returned to normal.

Jim