I just did a 3,000 miles trip using Lucas, 6-8 oz per tank. I yielded about 30 mpg traveling from FL to CA. Had a passenger and loads of stuff in the trunk and back seat. The trip was mostly highways, besides going into New Orleans, Houston, San Antonio, New Mexico, and Phoenix.
My point of view is that it lubricates the rings, cylinder walls, and valve seats. This will reduce friction on moving parts therefore increase engine response and most importantly, longevity. Less wear and heat.