I believe there is indeed such a thing as 'light cruise' vs 'heavy cruise'. Example that comes to mind is Interstate Highway 3 (H3) here in Hawaii. And don't get me started on the "Interstate" thing on an 'Island'.
Anyway, it's a modest grade and one of our local guys (Hi Cobra, Flip) has noted his 351 with T5 in OD 'struggles' up the grade at cruise speed. This is what I would call 'heavy cruise' load vs light cruise load. SO, he adds a little aviation fuel (readily available to him) to his premium fuel and is able to cruise comfortably with the higher octane rather than downshift. The higher octane is not required on the more flat H1 and H2 highways under 'light cruise' conditions.