FATZ, I don't know too much about '97 GM's. I'm going to assume it has a device called an ECU that controls the fuel to oxygen ratio.
If so, you might want to give it a little while when switching between grades of gasoline. It can take these things at least a little time to "learn".
Of course I could be completely wrong about this. I've been driving Nissans since 1991.
Out of all the bulletin boards in all of the world you had to walk into this one. Didn't ya.