I'll throw in my two cents on this topic. TonyP was the first person I know of to begin to connect the dots on recent engine oil formula changes and the corresponding trouble with wiping cams in flat tappet motors. Since his first sniff of this, I have done a great deal of research on this subject and just finished an article for a vintage vehicle magazine on this very topic.
In a nutshell, all of the posts above are spot on and all points are correct. The issues that concern me WITH BREAK IN are sufficient levels of ZDDP to form the necessary and historically experienced bi-metallic barrier on the cam in flat tappet engines. The new gasoline engine oils absolutely do not have sufficient levels to do this because of reductions to save catalytic converters.
Therefore, my opinion (and it is just that; an opinion) is to use Dello, Rotella, or Delvac diesel engine oils ONLY along with GM's EOS during break-in. I have converted 100% of my vintage gasoline engines to diesel engine oil for routine and that is my preference, but I can tell you that this engine oil world today is a whole new playing field from where it was only one year ago.
I have a lot of data from AERA (Automotive Engine Rebuilder's Association) along with a fair amount of technical data to back up my findings. HotRod did a great article on this topic too:
http://www.hotrod.com/techarticles/engine/flat_tappet_cam_tech/ So if you're running flat tappets, be very careful about what you do on engine break-in and more importantly cam break-in.
Each comment posted above from the other members in this thread is excellent and needs to be carefully considered. The engine oil world today is very different from what it was just a few years ago and all of it driven by conflicts with either emmissions equipment or emmissions standards.
