If the oil pressure gauge fluctuates over a wide range from zero to normal operating pressure, the most likely cause is...

The scenario where the oil pressure gauge fluctuates widely from zero to normal operating pressure is typically indicative of a low oil supply situation. When the oil level in the system is insufficient, it can lead to inadequate lubrication of engine components, resulting in erratic oil pressure readings. As the engine operates, fluctuations in the oil level due to oil sloshing or an insufficient supply can cause the gauge to display a variable pressure, sometimes dropping close to zero when the oil pickup runs low, and then returning to normal levels temporarily when the oil is circulated back.

In contrast, excessive engine wear would usually result in a more consistent drop in oil pressure due to increased clearances in bearings and other critical areas, not the erratic behavior indicated here. A faulty oil pump could lead to low oil pressure but typically would not cause wide fluctuations as seen in the gauge reading; it would more often yield a steady low pressure. A clogged oil filter might restrict oil flow and lead to a pressure drop but is less likely to cause such significant fluctuations as the system would mostly maintain a uniform low-pressure reading if the filter was the only issue.