Volvo Forums - Volvo Enthusiasts Forum - Need help adding an oil gauge

Have 2000 S80 2.8 T6 135K miles.
Want to add an oil gauge. Engine is still quiet but concerned pressure could start to drop due to o-rings.
Has anyone done this before? Need help finding the proper fitting to install a pressure sensor on the engine.
I am planning on removing the oil pressure switch and installing a tee fitting so the pressure switch will work along with the gauge.
Does anyone know the thread size and type of thread used for the oil pressure switch?
Thank you