Thanks to all of you for your help with this. I just ordered the Denso "front" unit from Rock Auto with the discount. The discount was good for $2, but every bit helps! I chose three day express service, so we will see what happens when it gets here. I'm crossing my fingers that the sensor will help increase my mpgs by like 4-5 mpg. I plan on installing this sensor, changing the fuel filter, and gutting the airbox to see what kind of mileage gains I get.

Oops yeah that didn't make sense. I'll clarify:

My o2 was NOT tripping a light, but I changed it anyway. Sorry.

In addition to the maintenance items already mentioned, you should check all the vacuum lines and fittings. I thought I replaced all mine when I got the car, but I missed one and was only getting mileage like yours. After I found and fixed the vacuum leak, I got 37 mpg on the highway the last time I checked.

Bastard,
I pulled the engine cover to check some of the vacuum lines, and found that two were cracked. I replaced them and I think I it made a little bit of a difference. Since this engine is new, you can be sure that one line is possibly messed up. I am posting a reply to your idea because I think its going to be very difficult to fix a vacuum leak seeing as I can't see air leaking. I tried the whole carb cleaner spray thing on my old car, but it did absolutely nothing. We'll see what this does. As of right now I filled up the tank to right where it said "F" on the guage, and it is now a shade above half a tank on the guage and i have put 60 miles on my car. This has to stop haha.

The ability to see air is not necessary to diagnose a failed vacuum hose. You already found two cracked vacuum lines, so it is reasonable to expect that there are more. Vacuum tubing is pretty inexpensive, I have found it best to replace all the rubber vacuum lines on a car when I find there are some that have deteriorated. I have also replaced several of the rubber 90 degree vacuum fittings because they tend to crack and leak too.

I'm curious about the current O2 sensor. I wonder if something happened to it when it was in the other engine. Wonder if the old engine was eating antifreeze and coated the sensor. I seem to remember something about this from a previous post. hope the new sensor is the answer for you

I just put the Denso unit in. I also gutted my airbox. The fuel filter will be done sometime next week. The old sensor didn't look like it was sooted or anything, it looked like it was in decent shape, but I suppose that might not mean too much. I suspect it was a bosch unit, it had three slits in the entire circumference of the sensor area. I believe thats what it looked like on the website to purchase the unit. It is my understanding that I will notice a difference after a few days of driving when the computer recognizes the new inputs and finally makes the changes to the fuel delivery.