My Saab specifies 87 octane (The US uses the R+M/2, which is about a 4-5 octane lower number than in Europe, Australia, Singapore; European 98 is about the same as 93 in the US) and recommends 90. Octane requirements are higher in hot weather.
I mix premium and regular to reach 90 in the winter. In the summer, I run it a bit higher than 90.0. Some of the last customer's choice remains in the hose so if they bought regular and you buy premium, you get some regular. One motorcycle forum said this was significant for motorcycles if they just buy a gallon. I first buy premium then pump regular second.
I found that gas mileage is slightly higher with 93, but that the increased costs cannot be justified by increased gas mileage. The US EPA tested a 2005 9-3 and published highway mileage is 1 mpg higher with premium. I'm not sure whether it was 0.6 or 1.4 mpg higher.
I found that in hot weather, the car has slightly more pep running on 93 than 90.
I rented a Dodge Charger a few months ago. The owners manual said the Hemi engine should use 89, 87 for the other engines. It also said not to use higher octane, just the correct octane.
In California, the highest octane that is readily available is 91. That might be why some cars specify 91, not 93. In Seattle, 92 is the highest found. 94 is available in Vancouver, BC.