Toyota 6 cylinders and I beleive Nissan and Honda have typically NOT required Premium, but if you push your engine they will perform better with premium. Usually the manual has a statement saying something like... "Premium fuel recommended, you can use regular, but you perfoamcne may be slightly impacted." My Acura TL unfortunately said Premium Required and even had it on the gas filler door. If you don't push your car hard the Premium will not buy you much, it is for when under heavy load (quick acceleration, steep hills/mountains) the engine will knock so the knock sensors will detect that and retard the timing, with premium the knock is not likely to occur and thus the engine timing will stay more agressive.