That's a myth. Synthetic can be used no matter what the mileage of the car.
It can only be problematic if the car is a frequent short tripper, or has direct fan/ no thermostat, or a clogged PCV valve, or too long a drain interval is used, a low detergent oil is used, or a combination of the above. These factors contribute to sludge build up; it can cause severe damage to the engine.
In such a car, dislodged sludge could potentially clog the oil filter, leading to oil starvation and failure of the engine. The only solution is to replace the oil filter several times before the next oil change. The synthetic's cleaning properties, combined with a few long road trips, will clean up the engine real good.