How could a turbocharger experience over-boosting due to cold oil?
The wastegate system uses engine oil pressure to drive the wastegate actuator, and this oil pressure is metered to the actuator by a controller that senses manifold pressure and throttle position and admits more or less oil to the actuator to drive the wastegate butterfly valve, to achieve the manifold pressure commanded by the throttle opening.

The responsiveness of the control system is sensitive to oil viscosity.  When the oil is cold, the controller and actuator is sluggish and laggy because of the thick oil, which can lead to overboosting when the throttle is opened quickly and the wastegate is unable to keep up, resulting in excess flow through the turbine and excess boost.