Take the intake off and put the stock one back on. Clear the codes. Does it still run lean?? If s o then its the Fitch. If not, then You have issues with the intake. check the inner diameter of the K&N at the MAF location and how far down the MAF sits. Contact K&N to get precise specs of what it should be. If the tube(too large) or mounting location is off you will go lean.
If i were you i would not drive the car too much with a lean CEL. You really risk frying your engine rings/pistons. The CEL basically means the ECU can no longer compensate for the difference in fuel required to run safely.
On to The Fitch Fuel thing... Thats a bs product to me. If it worked you would see car manufactures all over it by now.
Improved Fuel Economy (1 - 2.5 mpg) (fyi thats a normal difference so uou really cant tell if it works)
Increased Octane or Cetane (fyi impossible)
Improved Performance - Horsepower & Torque
Allows for the use of a Lower Octane (engines requiring 93 can operate on 91, those requiring 91 can operate on 89, & so on) (fyi any high octane car can and will accept lower octane, the ECU will adjust for knock)
Reduced Emissions (fyi impossible)
Reduced number of annual oil changes (fyi impossible)
Reduced Maintenance (cleaner injectors, carbs, plugs)
If you say this works then The Tornado for the intake obviously works as well.
PS pending where you installed the Fitch on your fuel line, i can reduce the psi of fuel to the injectors. Lean means not enough fuel. Just to make sure you understand, get a straw, blow into it. Now add a cig filter anywhere you want to that straw. Thats what the fitch could be doing.