The only help I can offer is that I would go over what you were working on first to make sure nothing came unplugged, unsealed, etc...before I went any further. (i.e.--EGR gasket still sealed properly, air-temp sensor still sealed properly, MAF still plugged in and sealed properly.)
I got a hunch that your 02 sensors are OK in that they are not at fault but are simply doing their job in indicating a lean condition.
The misfire though has me scratching my head 
I've had a vaccuum leak cause "multiple misfire" cell because of bad pinging.
Perhaps the fuel treatment dislodged some contaminents & possibly plugged fuel filter causing lean condition? Perhaps change-out fuel filter or at least pull it and see if you can blow through it (clogged).
When my Z-34 Monte Carlo started idling at 2500 rpm's it turned out to be intake gaskets leaking. Don't know if anybody here ever had high idle because of leaking intake gaskets? Doubtful that they would start leaking just like that, right after your service done though.
When my Fuel Pressure Regulator went bad, it affected mpg and resulted in starting issues (i.e.--after engine warm, like stopping for gas and having trouble getting it to run again..taking like 3-4 attempts to restart). No codes thrown though.
IAC is easily seen in throttle body throat...see if it looks carboned up...can be easily cleaned with o2 sensor safe TB cleaner spray. Although a bad one still would need changed out.
Hope any of these WAG's help.
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Originally Posted by taylorRichie
