I did a test recently.... With Shell premium I was averaging about 23-24 mpg, city & highway in my C280. I changed to Chevron premium (after reading about the purported benefits of Techron gas additives) and my gas milage dropped to 18-19 mpg city & highway. After switching back to Shell the mileage went right back up. Considering there are around 40 or so different gas "regions" in the US, all with different additives, etc. due to some states requiring oxygenated fuels, I guess you should just go with what gives you the best performance. BTW - does oxygenated fuel have any long-term negative effects on engine wear?
2 X 1997 C280