Take some of the oil and rub it between your fingers. It should smell like, well, oil. A crude oil smell, some oils like the old Castrol Edge (SM) have a sweeter smell like menthol (especially 0w20). If you have smelled a lot of oils and it is nastily different, you should probably not use it since there are so many GTX users on here that have not had this complaint.
You can always go to Castrol USA and have a "live chat" with some one.
I recently saw a Tech Bulletin from Castrol. Seems they are aware of the problem and have since corrected it. I don't remember the exact wording but it basically said the oil was ok to use. When I get back to work, I'll try to locate the bulletin.
For everyone else, it smells like really well used motor oil mixed with the normal fragrance of Castrol oils. Worst part is it hangs in the air for a good 20 minutes after its been put in a vehicle.