Synthetics last for so long that the breakdown of oil is not really a concern anymore. The primary reason to change a synthetic oil is due to contamination.

Modern engines are designed with much, much better tolerances than before and crank cases no longer vent into the atmosphere. If you use new synthetic oil in an old small-block Chevy, the oil will hold up for a long time, but it will be so contaminated by gasoline, moisture, and debris that it will still require shorter replacement intervals.