Winter gas has arrived. Winter formula gasoline has lower mpgs...
With a really high efficiency car like the Focus, the difference is more than 2mpg..
So figure you gained at least one for the oil, and lost three for the poorer quality winter formulation gas.
For all sorts of articles about "winter gas formulation" in Google.
It has something to do with the lower volatility gasoline (desired in Summer) portion costing more for, but that also gives a little better mileage.
So going back to Winter, the gains from that stuff is gone.
Raising the tire pressure should help a little.
I have my front at 44psi (the firmness in the front is less noticable due to the engine, and it give better handling) and 37psi in the rear. (any higher psi in the rear just creates a really harsh ride)
So your oil did not cause the loss in mpgs.. IMO anyway.