We use 91 in the 91S, otherwise it can knock slightly up a hill, but can use either 91 or 89 in the LS because that engine has knock sensors to change the settings. We use either 87 or 89 in the Milano, mostly 89 because it too will knock when going up a hill with the 87. On the level, no problem with 87, but of course, the Seattle area has many hills, some steep.
It is interesting that the required octane level at high altitudes, as in the Rockies, is a little lower because of the lower air density, In many areas, 89 is the highest octane available, and works just fine in the 91S.
Basically, all US supplied gasoline is "clean enough" regardless of additives which are similar to Techron or the like, etc, for keeping the level of deposits down both in the fuel system and during combustion, as Richard mentions. They do not degrade performance, but in the long run better ensure optimum performance from the engine. Using a higher octane level than recommended by the car manufacturer, without altering the programming of the ignition system, etc, however, does not increase the engine performance in almost all cases.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6