You can use any DOT4 fluid. I use Valvoline because it's available in the quart containers.
One thing I suggest if you don't have it already is the Power Bleeder. It's beautiful and makes complete brake bleeding/fluid change a 20 minute one-man job once the wheels are off. You can get one at
www.928gt.com just search for bleeder. The cap size is the same as the Boxster so it does not matter what year model is listed next to the bleeder.