A warm engine has low oil pressure. What is the most likely cause of the low oil pressure?

When a warm engine exhibits low oil pressure, one of the most likely causes is leaky main bearing seals. As the engine warms up, the oil viscosity decreases, which can exacerbate any existing leaks in the main bearing seals. These seals are critical for maintaining oil pressure within the engine by preventing oil from leaking out of the bearings.

When there are leaks in the main bearings, oil can escape from the pressurized system, resulting in reduced overall oil pressure. This reduction is particularly pronounced in a warm engine scenario, as the increased operating temperature can cause certain materials to expand, leading to an even larger gap and further exacerbating the leak.

While other options like worn valve guides or leaky hydraulic lifters can contribute to engine performance issues, they typically do not have a direct and significant impact on general oil pressure. Worn bearings can also lead to low oil pressure but are more commonly associated with a gradual decline in oil pressure rather than an immediate drop when the engine warms up. Therefore, leaky main bearing seals are considered the most likely and direct cause of low oil pressure in a warm engine.