there is
some truth to it... as I understand it, when synthetics were first introduced, they didn't have the seal swell rate figured in correct. in high mileage cars, if one were switching from conventional to synthetic, the synthetics would clean the debris that the conventional oils left and due to the slightly less amounts of seal swell and the condition of the seals, some cars would develop oil leaks from the slightly worn seals.
today, they have the synthetics zeroed in well and there are no issues from seals "shrinking" due to seal swell formulations.
when talking about a newer vehicle like the new gens, with relatively low mileage, it's safe to switch back and forth if one were so inclined.
if you trust the oil companies, here's a link to a similar question...
http://www.mobiloil.com/USA-English/...Motor_Oil.aspx
if you want to talk oil, this is a very informative forum to join...
http://www.bobistheoilguy.com/