How many miles are on these motors, folks? You really, really need to check the oil for coolant mixing in it. You may have experienced gasket failure due to dexcool. This is unfortunately very, very common. The dexcool is not compatible with the Nylon 66 used in the gaskets by GM and eventually causes failure. There is a class-action suit against GM over this since its so common.
See:
http://www.dex-cool.net/
The only way to fix is to replace the upper and lower intake manifold gaskets with aluminum-enforcements and quite often the upper manifold itself.
See:
http://www.pontiacbonnevilleclub.com...hp?f=53&t=3591