I don't have the Owner's Manual for your '94 model, but the '93+'95 Grand Prix Owner's Manual both say the same thing, so the '94 manual probably does too;
"Use regular unleaded gasoline rated at 87 octane or higher. It should meet specifications ASTM D4814 in the U.S. and CGSB 3.5-92 in Canada. These fuels should have the proper additives, so you should not have to add anything to the fuel.
In the U.S. and Canada, it’s easy to be sure you get the right kind of gasoline (unleaded). You’ll see “UNLEADED” right on the pump. And only unleaded nozzles will fit into your vehicle’s filler neck.
Be sure the posted octane is at least 87. If the octane is less than 87, you may get a heavy knocking noise when you drive. If it’s bad enough, it can damage your engine.
If you’re using fuel rated at 87octane or higher and you still hear heavy knocking, your engine needs service. But don’t worry if you hear a little pinging noise when you’re accelerating or driving up a hill. That’s normal, and you don’t have to buy a higher octane fuel to get rid of pinging. It’s the heavy, constant knock that means you have a problem."
Higher than 87 octane shouldn't be needed. If it is, to prevent knocking, there is something wrong with your car. And using a higher octane won't make anything else (like performance) better, and it may actually run worse, or get worse mileage, or both, if too high.
btw, vinny is right, although I might say 97% or 98% of cars. Depends on how you count. Only cars with higher than normal compression ratios, or that have a Turbo or Supercharger usually need higher than 87 octane.
thanks