Hi Karl There’s not much science behind what I can tell you but I hope my experience might be helpful. Our family live in north Scotland, almost exactly 450 miles from our home. In recent years we’ve visited them about forty times. Creatures of habit, our route, driving style and refreshment stops are reasonably similar each time. Out of curiosity I tried to assess whether the two petrol types made any difference and recorded the details of several round-trips, alternating the normal fuel with, what I call, the high octane stuff. Despite the extra cost of the premium fuel, indications were that the car consumed almost a gallon less over the 900 miles. That excercise was in a Volvo S60 2.5T but, equally unscientifically, I sense that our S Type also prefers to sip the Premier Cru.