Just a thought about the colder weather... I don't know about your Mustang, but my Integra will rev to 2000K at idle when first started. It will hold this rev level until it starts warming up; then the revs will slowly drop to 'normal' idle. I'm the type who fires it up, and drives. I suppose if you're the type to fire it up, and let it sit, that could affect your mileage.
I liked jsharp's thought about re-programming the ECU. But then I got to thinking it would probably straighten itself out withing five (5) fill ups.
Honestly, I wouldn't blame the oil... When you change multiple items at the same time, it's hard to pin a cause down to one culprit. Can you check to see if it's stored any codes? Like someone else said, maybe you've got a sensor going bad? I don't know a lot about the newer Mustangs, but speaking from a Honda point of view, when certain codes are stored, the ECU will go into 'Limp' mode... (so you can limp it to a garage I suppose). Basically it retards your timing, richens the mix, and does some other stuff. Just a thought. Good luck with it, eh!!