If there is the presence of a lubricant on a fastener, the coefficient of friction on the thread and the under-head changes.
When a fastening is then Torqued up, it is overcoming the friction and applying a clamp load. If the friction goes down due to a lubricant then the clamp load will alter for the same Torque applied.
It is important to maintain a constant friction to ensure correct clamp load. This is vital during assembly manufacturing to ensure correctly tightened Joints and avoid broken or deformed fasteners.
Crane Electronics, the force in torque management