Sudden (apparently) decline in gas mileage?

[nfpgasmask]

So last night I changed out the fuel filter. I also added some Lucas additive.

Now, to answer some questions, my spark plugs were changed just this past summer. Off the top of my head, I can't remember what brand. However, after I changed my plugs, I did not notice a marked decrease (or increase for that matter) in my gas mileage, so I doubt my plugs would be to blame.

Now, the O2 sensors and CAT I have been thinking about. With all that oil burning, they could potentially be full of soot. But, if an O2 sensors was flaking, wouldn't I throw a CEL code? I mean, I know our CPUs are intelligent and they "learn" to compensate for certain conditions, so maybe not?

My next plan of attack will be O2 sensors and FPR. I think we have two O2 sensors, right? One upstream and one downstream?

I will keep you all posted.

Bart