Quality of Gasoline?

As I understand it, it's not so much which refinery the gas comes from that determines the "quality", but the additives used and the condition of the station's holding tanks. Quality is marginally affected by the amount of water and other contaminants in the gas, which is in turn affected by the filtration. Each pump has filters which need to be changed regularly, and often are not, and the tank as a whole I believe has a filter as well. I don't think this follows brand lines so much as it does owners of particular stations.
http://ask.metafilter.com/mefi/23613

Selecting the best gasoline for your needs
http://advanceautoparts.com/english/youcan/html/ccr/ccr20030201fs.html

Quality of gasoline declines as prices billow
http://www.findarticles.com/p/articles/mi_m3306/is_5_115/ai_n14867128

Before You Buy Gasoline or Petrol
http://chemistry.about.com/library/weekly/aabyb100401.htm

The price and quality of gasoline
http://www.cockeyed.com/citizen/gasoline_prices/gasoline_info.html

Brand Autopsy
http://brandautopsy.typepad.com/brandautopsy/2004/07/the_gas_we_pass.html

Misc info
http://www.partsource.ca/doityourself/TIPS_fuel-efficiency-emission.asp

My top 18 chick flicks

Big (1988)
Love Actually [2003]
Thelma & Louise [1991]
Notting Hill [1999]
Smokey & the Bandit [1977]
Trust the Man [2005]
Bridget Jones's Diary [2001]
Under the Tuscan Sun [2003]
About a Boy [2002]
Dave [1993]
Steel Magnolias [1989]
When Harry Met Sally [1989]
Chasing Amy (1997)
Romancing the Stone [1984]
You've Got Mail [1998]
Pretty Woman [1990]
Four Weddings and a Funeral [1994]
Terms of Endearment [1983]