So I figured since the previous owner ran with the coolant temp sensor disconnected for six months and the spark plugs where completely fouled ( fubar'ed even ) that it would be a good idea to run some cleaner through the system ( I used liquimoly's gold label ) at the same time I reset the ecu after installing the new O2 sensor.
So I drove down to the south end of the Dead Sea from Jerusalem and really opened it up, and it seems like the mileage is significantly improved, though I won't know for sure till my next fill up.