I think the problem is more than likely your local station or branch of stations. For example, around here, all the Shell stations are owned by a small franchise known as Rose Oil Company out of the next county. The man who owns it probably has 25-30 gas stations in the tri-county area.

 

My past life I owned a 1988 Corvette. As has been posted Shell is a high quality gas as is Sunoco, Conoco. My Corvette was temperamental when it came to fuel. I stayed with Shell for the three years I owned her. She ran best on regular not premium. Back in the day that what was required for 5.7 liter L98. The 2001 Monte SS gets a steady diet of Shell or Conoco.
No enthanol gas mix. There are alot of different brands around the country. It is wise to shop for a good quality gas.

Donald