Is this a new rebuild?

The story goes that Zinc was removed from oil and this said zinc is needed in the critical break-in period

I read in grassroots motor sports how David Vazird cured a problem on a GM truck that had lifter clatter using Royal Purple oil

On cam and lifter changes proper break-in is essential
Now more than ever since they removed the zinc

http://royalpurple.com/breakin-oil.html

HTH
David