With the engine at idle power, a red OIL PX message illuminates when oil pressure falls below _________ psi.

The correct answer is 15 psi. In the T-6B aircraft, an oil pressure warning is triggered when the oil pressure drops below this critical threshold while the engine is at idle power. The oil pressure system is vital for the proper lubrication of the engine, ensuring efficient operation and preventing damage from inadequate lubrication.

An illuminated red OIL PX message serves as a warning to the pilot that oil pressure has fallen below a level that could lead to engine failure if not addressed promptly. During normal operation, the oil pressure should remain well above this level, indicating healthy oil flow and adequate lubrication. If the pressure does fall below 15 psi, it is a serious concern that requires immediate investigation and corrective action to prevent engine damage.

The other pressure values mentioned are thresholds for normal operation, and they do not correspond to the critical warning level for the red OIL PX indicator at idle power.