Just purchased another GN a couple of months ago. The previous owner had the dealership replace the rear main seal. I started noticing a small leak come to find out it was the rear of the oil pan gasket leaking. someone had over tightened the oil pan and destoryed the gasket. I have a replacement gasket coming from Kirban's. If it is the rear main seal leaking I'm not aware of any quick fix...just make sure it is not the gasket leaking.
HTH