No sure where you live Dan, but where I am there are two giant tanks
down at the port. Not sure who owns them doesn't matter much because
all the oil company trucks pull up to the same two tanks and fill up.
The only difference in the delivered gas is the additive package they
put in.

Aside from octane and cleaning agents, I don't think there is anything
potentially different.

All the Clark stations in my area (northeastern Illinois) changed to Citgo
or Marathon stations, or were bulldozed about a year ago shortly before
their parent company, Premcor, was sold to Valero.  See www.clarkretail.com 
and www.valero.com for history information.  I expect that the gas they use
would depend on the station's location.