I have a 99 Firebird with the same engine, minus the supercharger, and I really like the Pennzoil High Mileage 10W-30 I'm running in it right now. It worked great at slowing down a slight leak it had and it sounds very quiet as well. I've driven the car in a few cruises (stop and go for many hours at a time with jack rabbit starts and some aggressive driving/revving/etc.) and it's held up great. I tried Mobil 1 High Mileage 10W-30 before this and performance wise it felt the same, but it did absolutely nothing for the leak. Plus, the Pennzoil is over $5 less per jug.
Edit: forgot to mention that in 1999, GM recommended 10W-30 for these engines. I'd go with that.