There is debate on Toyota's OCI for the 5.7L Flex fuel vs non and no one knows why. From my POV, if I am not running E85, then I can go 10k easy.
My basis for such an opinion is the many UOA reports out there, many of which have gone near 10k (FFV and Non) all look stellar. UOA's done at 4-7k all say to run the oil longer, and some even looking like virgin oil.
THERE IS NO difference found on any UOA I have come across showing FFV motors are harder on oil than Non-FFV.
I have yet to do a UOA on mine, but I can tell you I recently changed the oil after a full season of towing and about 6k miles total, and it hardly looked dirty. Granted "black" colored oil doesn't mean squat, clean looking oil is likely "clean" and would result in a great UOA.