I fitted an SPA dual Oil Pressure/Oil Temperature gauge to my M3W back at the end of 2014. I did do some posts on it then but the pictures are no longer retrievable.
Unfortunatley I have now sold the car so I am not able to show a picture of where I picked up the power from.
It should obviously be continually live whilst the ignition is turned on and there is a fairly thick wire (which from memory was yellow) coming from the ignition swicth and I connected the power feed (Red?) cable from the guage to this cable using one of the Insulation Displacement type 3M cable connectors. The gauge draws very little current so this is more than adaquate.
The sensors for the gauge are mounted in an aluminium billet sandwich plate that mounts between the oil filter and the engine.
You will also need the threaded adaptor that holds the sandwich plate in position and allows the oil filter to be screwed on up against the sandwich plate.
The gauge, sandwich plate and threaded adaptor are all available from Merlin Motorsport (at Castle Coombe) in the UK
I found this set up particularly usefull because you get a real-time reading of exactly what the oil is doing with regard to temperature and pressure..
Good luk with your installation
