I haven't been able to find the original document but a number of forums link to a GM Service Bulletin on the subject. Here's one:
GM debunks ZDDP myths for flat tappet cams - Bob Is The Oil Guy
Comp Cams disagrees:
http://www.compcams.com/Base/pdf/FlatTappetCamTechBulletin.pdf
GM debunks ZDDP myths for flat tappet cams - Bob Is The Oil Guy
Comp Cams disagrees:
http://www.compcams.com/Base/pdf/FlatTappetCamTechBulletin.pdf