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p-wagen
When I first got my Boxster, 93 octane Texaco was available. When they pulled out of my area, I went with Shell, 91 octane. (Highest octane available). Now they have left and those stations have changed to Phillips 66. Other Top Tier fuels available to me are Conoco and Exxon. Any recommendations on which of these three would be best for the Boxster?
Over the years and miles I've run Chevron, Shell, Texaco, Phillips 66 and Conoco in both of my cars (all of my cars actually) with no differences noticed except for one I'll get to in a bit. (Exxon is not listed as I can't recall ever using it as the other brands are just more familiar to me and I naturally gravitate towards one of the more familiar brand names when it comes time to buy gasoline.)
Of the 3 brands you have available, Phillips 66, Conoco, and Exxon, Phillips 66 would be my first choice all other things being equal as there was a 66 station just a couple of blocks from my house and I spent a lot of time there watching the two owners/operators -- brothers -- working on cars. They treated me very kindly and let me observe without shooing me away.
However, ignoring any preference not based on emotion, any of the 3 brands gasoline should be just fine. In the outside chance the car reacts unfavorably to a particular brand try another brand. This is what Porsche advises.
You don't have Chevron available to you but if you do have a brand of gasoline that offers Techron -- I think Texaco is the only other brand to offer Techron -- I would use that brand of gasoline. I found that after a couple of years of using Shell almost exclusively that when I switched to Chevron -- the Shell station raised its prices quite a bit over the prices charged by other stations in its immediate vicinity -- this switch to Chevron after a couple of tankfuls caused a noticeable improvement in my performance of my 2.7l Boxster engine. I attribute this to the cleansing of deposits by the presence of Techron. Shell gas would all manifest this noticeable improvement in performance but only after a longish drive at highway speeds. Chevron gas did this during my normal usage which is nice as arranging time for longish drives was getting harder to do.
Whichever brand you settle on some rules: Buy the highest octane gasoline you can find -- barring race gasoline (unless you are going to the track). 93 is best but if all you can get is 91 so be it. Buy from a busy station to ensure the gasoline you get is freshest. Busy stations get their tanks refilled more often -- I've come across stations that get their tanks refilled sometimes several times a day during real busy times -- and this helps insure the gasoline is fresh. I try to avoid filling up when the tanks are being filled, but a number of sources have told me the filtration systems at the pump prevents any sediment stirred up by the refilling being carried by the gasoline into the car's tank. Still, old habits die hard and this has been my habit ever since I have owned a motorcycle/car.
Last but not least try to avoid running the fuel level real low, avoid making a habit of this. The fuel pump resides in the fuel tank and is cooled by the fuel. Thus I do not make it a habit of driving to the point the fuel tank level gets real low, though I admit sometimes -- not often but sometimes -- driving until the low fuel level light comes on. By running the fuel level way down this results in the fuel pump running warmer than it would otherwise. It might not matter but it just feels "right" to me to try to minimize the time the fuel pump is exposed to low fuel level in the gas tank.