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the gas milage imporves as the engine breaks in.
SIDE NOTE: switching to synthetic before 5,000 miles is a bad idea.
I've never gotten less than 20 mpg and i drive through rush hour traffic every morning. I'm very happy with my gas milage, it high for a 2.5L motor. Keeping in mind that i shift at about 2,500 and try to cruise at 2,000 rpm.

 

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RE: changing to syntetic. If you ahve already made the change to synthetic don't drain it out and go back to regular oil, that syntetic is to damn expensive to do that. Once you've changed stay with it.

RE: reason not to change Durring break in the engine needs to develop its seals, if you use syntetic which is a light weight and thin oil it takes longer and is harder for the engine to develop these seals. This is based 1/2 in fact and 1/2 in opinion. However i have loads of experiance with motors so i'd say its a well researched opinion.


EDIT: its stil possible to break in an engine without damage while using syntetic, just keep a very close eye on your oil level and drive friendly your fist 5,000+ miles.

 

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also quick update on gas milage (i've tracked it in a log book from day one):

lowest: 20.468 (4th tank of gas)
highest: 27.869 (all highway from houston to dallas)
average for weekly driving to work (30 miles round trip a day):23.512

hope that helps... oh thats in Houston with a box K&N filter and valvoline 5w-30 regular oil.