[Dave S.]

In the US, available octane in the various gasoline grades changes with altitude. Since the air is less dense at higher elevation you don't need as much octane to prevent detonation. That's why all 3 grades have less octane as you climb in elevation. I assume the 93 number recommended by Porsche is based on seal level elevation. You can't even buy 93 where I live at 8700 ft.