Most Grand Prixs with the Series II 3800 engine were delivered with AC Delco platinum spark plugs from the factory. 41-921 which are double Platinum meaning both the ground electrode and center electrode are platinum and should last to at least 100K miles.

Here are the colder AC Delco alternatives:

AC-Delco "double platinum" plugs:

Stock..........41-921
1 colder.......41-932
1-1/2 colder..41-919
2 colder.......41-913

Starting gap all at 0.060"

Note some GTPs circa 98-00 may have been delivered with NGK PTR4B-15 double platinum spark plugs in lieu of the AC Delco 41-921. Colder ranges of NGK are unknown.