What is the most probable cause of small fluctuations in oil pressure?

The most probable cause of small fluctuations in oil pressure is the pressure relief valve sticking. The pressure relief valve is a critical component in the lubrication system of reciprocating engines as it regulates the oil pressure to prevent it from exceeding safe limits. When this valve sticks, it may not open and close properly in response to changes in pressure, leading to inconsistent oil pressure readings.

Fluctuations occur because the oil pump continuously pumps oil, but a malfunctioning relief valve could cause the pressure to spike temporarily until the valve reacts, then drop again as the valve inconsistently releases pressure. This intermittent sticking results in the small variations observed in oil pressure readings.

Lack of oil would typically lead to lower oil pressure rather than fluctuations. Air in the oil tank could create a foamy mixture, but this would generally lead to a steady drop in pressure rather than fluctuations. Lastly, if the scavenge pump is working at a higher capacity than the pressure pump, it would more likely lead to problems with oil supply and consistent pressure rather than subtle fluctuations. These distinctions help clarify why the pressure relief valve sticking is the most likely cause among the options provided.