I know this doesn't answer your question, but they are actually an Alliance Vendor (if you didn't already know) ...at the bottom of the list: http://www.jalopyjournal.com/?page_id=176 Malcolm
You may be able to purchase this locally, at a GM dealership: http://www.gmpartsdirect.com/results.cfm?singlepart=1&partnumber=EOS
A recent one: http://www.jalopyjournal.com/forum/showthread.php?t=343217&highlight=zinc+oil&showall=1
Just run a good brand 10/40 or 20/50 oil. All the ZDDP you will ever need is in them. You do not need an additive.
put the ZDDP in your motor , i have seen several camshafts go flat in motors that had a good quality motor oil in them