My impression is that the `no break-in' thing with synthetic oils comes from
the lack of lighter fractions in the `pure' viscosity synthetic.  There were
a lot of oil leaks when fully synthetics were first introduced.  It was
discovered that the o rings and other `plastic/rubber' bits need some of the
lighter oils in order to swell up and seat properly.   The purely synthetic
oils did not have these impurities hence the seals did not take up these
lighter fractions and so the oil leaks.  Take a new engine run on mineral
oil and let the seals swell up and then change to a pure synthetic was the
advice I heard (FWIW)....
Mike