High altitude driving is usually not a problem, as the engine doesn't require as high an octane rating gasoline there. That's why when you travel through the Rockies (at +4000 feet), etc, the octane ratings at the pumps are lower by a couple of counts.
My 91S engine doesn't seem to like (making rattly sounds) anything less than 91 on many of the steep hills in the Seattle area. Flat lands might be ok.
The LS, with it's knock sensors, can handle 89 ok, but I put few miles on it, so I run 91, what the heck.
Our Milano, though, doesn't like 87, also making rattly sounds on hills, so we run 89 in it to quiet it down.
89 Milano (wife's daily driver since 1989, Shankle Sport)
91 164S (my daily driver since 1994)
94 164LS (~Q) (trip Alfa since 2000)
72 Morgan 27 (water time since 1976)
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