90 911 Oil Pressure problems?

[CountryHill]

we have a 1990 911 carrera 2 automatic with some oil pressure issues. This all began with some broken cylinder studs. The technician removed the engine to replace these studs and clean up the heads. Now my problem is the engine has about .25 bars of oil pressure at idle when engine oil temp reaches operating temperature. The technician had removed the oil pressure regulator when disassembling the engine and believes he put it back together correctly. We're running castrol gtx 20w50 in this car. Does anyone have any ideas as to why we would be loosing oil pressure? Any assistance would be awsome.