In all lies is a grain of truth. Early in the days of synthetic, the compounds in the oil had different effects on the internal seals of an engine. Petroleum contained distillates that caused the engine seals to expand or "swell". The seals would wear to the correct tolerance for that component and everything would be fine, until the owner switched to synthetic oil . Synthetics did not contain the distillates and the seals would return or "shrink" back to their original size.
This caused sometimes incredible oil leakage issues. An interesting side bar is the invent of several "stop leak additives" to battle these problems. The primary component of these was and still is today.... Petroleum Distillate
If an engine had synthetic oil in it from the beginning and for the majority of its existence, the seals would not swell and wear until the owner switched to conventional oil. Once the switch was made, you could never go back due to the extended wear to the motor.
Both synthetic and conventional oils are "engineered" today making them very similar in detergent and distillate levels, so switching is not so much of an issue anymore.
This guy sounds knowledgeable but I would feel more comfortable if someone could confirm this, thank you!