This an old myth. And it is just that, a myth.
If you have an engine with high miles and poor maintenance, you will have sludge. Sludge will actuall mask degraded seals by plugging up the leaks, when you introduce synthetic oils with extremely lage amounts of detergents in their additive package, you will be shown where your seals have degrade by finding leaks.
If you have an engine in good repair, maintained properly, with servicable seals, you will not find leaks after the switch, no matter what the mileage. This applies to all quality synthetic brands.
Some of the oldest ester based synthetics did attack certain types of gasket/seal material, these formulations are to the best of my knowlege gone by about 15-20 years or so, and there is only one company still using ester base stock in their synthetics AFAIK.
Amsoil uses a full group IV PAO base stock, and actually has seal conditioning additives in the additive packages to help better maintain seals and extend their service life better than other brands.
Hope this helps, let me know if I can answer any other questions.
Ashton