This is sort of a "mechanic's myth" that's couched in a little truth. Basically, most true synthetics will dislodge and clean the engine of deposits built up over time from the use of regular oil. This build-up has been acting as sort of a "plug" behind the already worn seals and gaskets in an engine, which when removed allows seeping and in some cases, outright leakage. The culprit isn't necessarily the synthetic oil, but the underlying wear it subsequently uncovers. Also, engines with more wear can sometimes have a little blow-by which will also tend to consume oil. Couple it all together and you can see where that story comes from.