A modern 'generic' engine from any current vehicle maker with electronic fuel management should be able to cope with any variation in fuel grade between about 90 and 100. I don't think any car maker builds cars without at least a basic ECU these days, mainly because the engines can't run properly without them. 8-)
My older pre-ULP (one 1983 model and one 1985 model) Saab C900's run just fine on premium fuel because the grading is between 95 and 98 RON and the engines are made from the right materials to deal with the different additives. I have not been able to find out whether premium fuels have valve protection additives in them though, so every few tankfuls I'm adding some Flashlube just to be sure.
Saab's 8V B201 engines (fitted to Saab 900's built from 1979 up to 1988 -
16V engines started being an option in 1985 btw) were designed to run on leaded fuel graded at about 97 RON, so the phase-out of LRP has forced the investigation of other options. The engines will run on lower octane (normal ULP, etc.) but they don't have any electronic fuel management (it's all based on the Bosch CIS aka K-Jetronic system) so there isn't any way to 'condition' the engine controls for different fuel grades.I personally use Optimax whenever possible - other premium fuels give mixed results (BP Ultimate was the worst btw!).
One other reason that I think the B201 Saab engines run better on higher-octane fuel is because they produce most of their power in the 3000 to 4000 rpm range.
Regards,
Craig.