Thanks again, J cAT. Great advice!
I'm still undecided as to how to approach this whole warranty issue. If the fluid is black, and I change it, will the dealer claim I had maintenance performed without his authorization and thereby void my warranty?
I think I'll just see how the fluid looks. If it's clean, that's great. I can live with that. If it's black, I think I'll phone the dealer immediately and inform him. I don't think any reasonable person would prevent me from changing the fluid in that event.
I think I'll go out and get another Onstar diagnostic right now.
Thanks again!
Bill