Not sure if they have Sunoco gas stations in Seattle, but the east coast Sunoco pumps have 4 options: 87, 89, 91, 93. 91 works well in my 560SL.
Higher octane additives are put in the fuel to suppress engine knock or "pre-detonation" in the cylinders. Contrary to belief, "premium" fuels generally are not better qualty gasolines, but actually reduce the volitility of the fuel with additives to prevent pre-ignition. High compression engines compress the fuel/air mixture to a greater extent so the concentration of fuel is higer in the combustion chamber. Hot spots in the chamber (like a bit of hot carbon) will ignite the mixture before the plug fires. When the plug fires, the the two burning fronts collide and knock occurs. Not good for the pistons. Rough idle is rarely fixed by premium fuels. The Techron route is better.