Losing oil pressure ('91 100)

[Andy Schor]

A mechanic/friend asked me to post this for a customer's car.

The car is a '91 100 n/a (auto) with ~160k miles.
Once it warms up, it will not hold oil pressure (drops to ~10lbs).
He has replaced the oil pump, took the head off  and checked the 
gasket for tears, checked the jets below the pistons, oil cooler,
bearings, etc.

It does not leak oil, nor is it burning oil.

Any BTDTs are greatly appreciated.

Andy Schor
'90 CQ