Actually, the superchargers used in the GTP (and most other GM's that have 'em) does have an oil change procedure. Inspect every 30k and top off, and change every 50k. There is a drain bolt (and it also serves as the filling location) on the middle part of the supercharger body.
I know the dealer parts dept has the supercharger oil, you'll need about 8oz. Not sure of the cross reference of the GM supercharger oil to something off the shelf.