In addition to losing 3 mpg by check engine light comes on and off. it's staying on longer each day. I am going to bet it's oxygen sensor time. How many are there ( 2 or 3), and how hard are they to change? Cost?

Should I get genuine FOMOCO or part store brand?

If its pre-96, it is EEC-IV and has 2. If it's 96 and above, its EEC-V and has 4. What year vehicle do you have. Also, the on/off light signifies a problem that appears then disappears. The computer will have a code, so get code and go from there. All of the major parts stores will read it for free.

What he said.....

Should I get genuine FOMOCO or part store brand?

Buy only stock Ford OEM. That cheap Bosch sh!t is just a money burner...

Mine Is A 93. How Difficult Are They To Change? Should I Just Change Both At The Same Time?

Maybe somone else with a 93 will be able to post a pic, but if its like the 2 front sensors on my 96, its going to be a bitch.

Edit, before doing that, you need to get the code read. You need to see what the computer see's. There are quite a few things that will reduce gas mileage.