My mpg has dropped by over 10% in the last couple weeks due to a dramatic drop in temperature from the 80s to the 40s but yours is quite a bit higher. Cold plus the wrong oil still wouldn't add up. First thing I think about when I hear such a story is injectors. Anecdotally, sometimes that is the issue and no error codes result to pinpoint the problem.
So I'd check it out with your dealer, you will probably have to raise a fuss to get this resolved. I'd ask them about oil as well. I brought my own oil to the dealer for my first service. They claimed to have the right weight of oil (0-20) and that it was synthetic, but when I found out it was bulk instead of containers I could verify, I used the oil I had brought just in case. I guess I have trust issues! I could have asked to look in the service department at the (I'm guessing) drum they used, but I had this mental picture of a unlabeled nozzle from some back room with no way to trace it out.
My first tank in Feb this year was 42 mpg. Despite similar weather in the next few fills, I've never gotten below 49 mpg since. My 14,000 mile average is over 52 mpg if that helps. That is every drop measured, not the highly variable displayed mpg. If you have the 18" wheels, 43 mpg in cold weather is actually not bad.
2018 Kia LX HEV Metal Stream with Advanced Tech