Considering all the variables of vehicle operation, including...
Driving style
Fuel composition
Operating environment
Engine design
Engine reliability
Lubricant additives
Routine maintenance
Aftermarket performance upgrades
(Etc.)
...I find it harder and harder to extrapolate anything from first-hand accounts of oil "performance." The brand of oil, the specific product of that brand and the weight of the oil are just a small part of the overall picture of what's going on with your engine. For instance, with longer OCI's, how can we tell if it's not a seasonal formulation change in your gasoline that isn't contributing to the poorer performance? Or perhaps it's something simple like using the A/C more often now that it's summer? Or maybe it's the combination of the seasonal gas and the A/C, combined with a new driving route due to road work and perhaps the fact that you're approaching a major service interval for your vehicle?
BTW: Timeau, I didn't mean this "rant" at you in particular (or anyone else for that matter). I'm just sort of commenting in general upon what I think is an overemphasis here upon brand preference. IMO, I think people have to go pretty far out of their way to pick a poorly performing oil and experience a reduction in performance; the forumulations of major-branded oils are so good these days that it's almost a "quality commodity" product. I don't know what is causing your reduction in mpg, but I suspect it's a combination of small changes in disimilar things rather than just a simple change in oil brand.
