Brian,

Your car, like my 86 300E, doesn't have any type of "knock" sensor to detect preignition knock and retard the timing to prevent it. You really do need to run premium in the older cars. You may experience some knocking even with premium on hot days especially under low to moderate rpm/high load situations. Avoid this as much as possible as preignition (basically, the "explosion" of the fuel/air mixture in the combustion chamber rather than the controlled rapid burn that it should be) is very hard on the engine components. It's been known to hammer or melt the aluminum piston crown until it blows a hole right though it, with rather disastrous results, as you can imagine. It might even be a good idea to run an octane booster since they require 92 octane minimum and the lowest (at least here in CA.) seems to be 91 octane.

Gary