I have been using GC in my Continental for the past couple years or so. Gas milage has not gone down. Still gets great. On long trips, still gets 26 to 29, averaging 27. Got the same milage on trips back when I would use regular Group II oils, but I keep this GC Group IV oil in for 10,000 OCI.
When someone says they are getting worse milage with brand x verses brand y, it is probably because the type of driving, and the season was different between the two.
Presently using PP 5W-20 (the older Group IV, not the newer Group III) in my MKZ. When my stash runs out, I will switch over to the GC 0W-30, as long as it stays Group IV, Made in Germany.