I would physically clean the "grease" that has formed as a result of the leak. Becareful about what you use to clean it. I would suggest something like simple green, in small quantities, and a small amount of water to wash away the loosened oil. Again, be very careful..and be patient. It will likely take many applications.
Once cleaned, you can monitor the areas...and will likley spot the culprit. If its the valve cover..its not a difficult repair...just a pita due to the location.
I wouldnt use synthetic while undergoing a auto-rx treatment. A good quality dino will do nicely. After the treatments are finished, then go to a quality synthetic.
BTW, many people have found that their oil leaks have stopped after the auto-rx. Do a search on the subject over at the other site.
Also, with about 126k on your car...You will need at least two bottles of auto-rx. There is a link that is a step-by-step process in this treatment. I'll post it later when i get home.
Supertech is wally, I believe. Its adequate, but nothing more. I'd try wix if you cant get the others. Napa used to carry wix, but cant say if they still do.
1999 Buick Regal LS
ZZP 2.5" SS 304 DP, GTP Cat Back , GMPP Handling Kit, poly links and midpoints, HIR 9012/9011 with upgraded relay harness, 5000k HID fogs , 12" Impala/MC brake upgrade, Mag cat 94106, Ported and Polished LIM