Check out AOPA's ask the A&Ps monthly podcast series. It seems to me that about 1/4 of the questions are about new engine or new cylinder break in.
Consensus is DO NOT use synthetic oil. Do NOT use anything with antiscuff or antiwear additives. Run the engine hard until you notice a sharp drop in temps - might only be a couple of hours if you do it right. And change the oil early. 10 hours max.
I took notes someplace with specific temp numbers for Continental and Lycoming temperatures before and after the rings have seated and the engine is broken in, but the podcast is very detailed and very specific on break in procedures.