What is the most likely cause of excessively high oil pressure?

The most likely cause of excessively high oil pressure is a defective pressure regulating valve. The pressure regulating valve is designed to maintain optimal oil pressure in the engine by allowing excess oil to return to the oil pan when the pressure exceeds a preset level. If this valve is malfunctioning or defective, it may not open when needed, causing oil pressure to build up to excessively high levels. This can lead to potential engine damage and increased wear on components due to inadequate oil flow to critical areas.

While a clogged oil filter can cause pressure issues, it typically leads to low oil pressure due to a reduction in oil flow. Low oil viscosity can also affect oil pressure, but in the case of low viscosity, it tends to decrease the pressure rather than increase it. Excessive oil levels can cause pressure issues as well; however, a defective pressure regulating valve is a more common source of failure and will directly lead to sustained high pressure which is beyond safe limits.