[s-cars] High oil pressure

[Radek]

My son's 1996 UrS6 has developed a problem:  an oil pressure light/beeper accompanied by dropping tachometer (once the engine is started, the tach almost immediately  drops to 0).  Assuming an electrical issue, he replaced the black oil pressure switch - no change.  Next, he connected an oil pressure tester gauge in place of the sender.  Started the car, the needle immediately went to 90 psi.  Seems way too high, even though this was on a cold engine.  According to Bentley, max pressure shouldn't exceed 60 psi.  Removing and cleaning the pressure release valve (behind the oil temp gauge) didn't change anything.

Any suggestions where to go from here?
Thanks.

Radek.