Can it use 87 octane or must use premium?
Do you know the compression ratio of the engine? If not, do you know what engine it has?
It is actually best for your engine and performance to use the lowest octane possible. Octane increases the fuel's resistance to ignition to prevent pre-ignition and detonation in high-compression engines.
If your Grand Prix is a GT, it only needs regular(87). If it is a GTP, you must use premium. GTPs (supercharged) after 2003 could get away with regular (pemium is preferred), but it could hurt your mileage and performance.
If you run a high octane engine running a lower octane is really bad for it so don't "always use the lowest octane possible". Use what your car is rated for. If you put a higher octane of gas in a low octane engine you'll just be wasting money, no benefits.
Looking for a Used Grand Prix in your area?
CarGurus has 3,703 nationwide Grand Prix listings starting at $879.
Search Pontiac Grand Prix Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Pontiac Grand Prix Experts