All of the following can cause low oil pressure EXCEPT:

Low oil pressure in an engine can occur due to various factors that affect the oil's ability to circulate properly. Among the potential causes listed, worn pistons do not directly contribute to low oil pressure. Instead, worn pistons typically lead to increased oil consumption or excessive blow-by, which affects compression and potentially the overall efficiency of the engine.

On the other hand, diluted engine oil can lower the viscosity of the oil, causing it to lose its ability to maintain pressure effectively. Excessive bearing clearance creates a larger space for oil to flow, which can also reduce pressure. A clogged oil pump pickup inhibits the flow of oil to the pump, leading to pressure drops.

Thus, the condition of the pistons, while it can influence engine performance, is not a direct cause of low oil pressure as the other options are. Understanding the mechanics of these components can help you diagnose oil pressure problems effectively.