Hello and welcome.My average mileage before the oil change was around 27 mpg. Since the oil change its averaging around 23 mpg. Needless to say its disappointing.
It does not really make much sense - how come oil change would affect your fuel consumption? Unless completely wrong oil was poured in... which I doubt.
I use Pennzoil Platinum 5W30. I cannot say much how better or worse it is than others as I did not use anything else. However, I can tell you that there is a huge difference especially on cold. Years ago, when I switched to synthetic from mineral, I immediately noticed easier cold start and less fuel use on cold. It took more for the engine to warm up, though. Not a lot, but noticeably longer.
As for your case. Are you sure it does not have to do with your driving style and weather? It got much colder than it was 3 months ago. This car hates cold weather. Fuel consumption is much higher on cold days - the time to get engine to operating temp.