I'm afraid in Calif. they are all the same, with some minor exceptions that really don't make any significant difference. Exxon no longer exists in California, so I'll try the next best thing, which ought to be either Chevron or Mobil. I just put a bottle of Techron Fuel System Cleaner in with the last fill-up, so I'll see if that helps at all. I'll check the timing this weekend. Any idea what amount of spark advance I should see under free revving, no-load conditions?