Will changing to synthetic engine oil damage the seals?

Owners of recently purchased pre-owned cars often want to start off with a fresh charge of synthetic motor oil, but are worried that synthetic engine oil damages seals.

Many people believe that switching a higher mileage engine from a conventional oil to a Group IV synthetic polyalphaolefin (PAO) can lead to seal leakage: but is there less inherent risk with switching to a Group III oil? The rational here