Like Tom says, EOS is one of if not the best sources of zinc on the market today. Problem is it can only be bought from a GM dealer at a cost of $7 a pint afer a resellers discount.
As far as synthetic oils go, one of the best is the Mobile 1 V twin oil, which is loaded with zinc. Most all the other oils, except for racing oil and just a few others are LOW in zinc content. There was a post on another board that has a link to a site that gives the chemical breakdown on most of the major oil brands, both syn and dino and Rotilla is also a good choice.
Local guy builds Hooters Pro Cup engines and swears by EOS for break in on those engines. I also just started running that in my Cummins with Mobile1 syn.