Gas brand?
 

My car seems to hate Sunoco 91 octane gas... it acted better with Shell premium. Anyone else have this happen?

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?

Shell has recently come out with a "new" gasoline. Marketing gimmick? :D

25% difference in MPG as mentionned by the previous poster is highly/very/almost improbable, but probably impossible.

Remember that the gas you'll get at one station might not be the brand appearing on the street sign. There are thousands of exchange agreements between the gasoline retailer/refiners where, as an example, Shell (or any other refiner) would supply a certain region and in exchange, Cheron would supply in other regions. gasoline is gasoline is gasoline
All gasoline in a specific region must meet the local regulation and nobody is giving freebees. They meet minimum regulation.
High octane for a car that does not need it is a total waste of money. You'll get no benefit in term of MPG, power, or anything else.
Check this post for more info.
http://www.peachparts.com/shopforum/tech-help/45279-premium-gas.html