1996 was the first official year of OBDII but some cars started earlier as that current design would be used in 96 or later. My 95 Neon was built in Jan. 94 and my scangauge works perfectly fine.
You should have no problem with a 98 grand prix, but there is a list on the website with cars its known to work with if you want to double check.
For that car, get the XGauges for transmission temperature and knock retard at a minimum. I also have both O2 sensors and long-term fuel trim programmed in on the gauge in my 99 LeSabre.
If you have the 3800 Series II naturally aspirated, my car has the same engine and transmission as yours, and these have worked flawlessly in my car.
Have a SCII in each of my vehicles and I love it. I love monitoring sensor readings right in front of me as I drive.
I do too. I have my SG-II in the Camry Hybrid. I'll need to get into the X-guage thing, as there's so much more info on tap that I'm not seeing. So for, though, after nine months of ownership and about 25k miles, not one single code has appeared to be "scanned" -- can't say I'm too heartbroken about that, but at least I know what to do if one appears.