I just changed the oil in my '05 GXP. This was my second change at 6100 miles, and both times I put in 8 quarts and changed the oil filter.
I put in 7 quarts and then start the engine to fill up the oil filter. After I do this, the dipstick reads about 2/3 down in the safe range. I then add the 8th quart (of Mobil 1 no less), and the dipstick read almost completely full.
I had a '67 Bonneville 428 as my first car. That thing took 7 quarts when the filter was changed, and I thought that was a lot. The Northstar has it beat, but I hope that the extra oil capacity translates into better lubrication and longer engine life.