Bad Oil Pump?
 

Original owner 350SDL 159K miles. Oil pressure almost nil at idle, just over "2" on gauge at higher RPM. Pressure drop happened suddenly. Engine is still running fine with no abnormal sounds other than lifters due to low oil pressure.

My mechanic recommends oil pump replacement. I hope it's not more serious. Anything else we should be looking at?

Thank you.

The mechanic's diagnosis sounds right on. You might want to use a test gauge to confirm the low oil pressure, but I wouldn't run the engine too long in its present condition or you'll be replacing more than the oil pump.