For 71-73 Pinto's, both the engine oil pressure and temperature sensors are connected to that instrument cluster light bulb.  The light bulb is lit by either senor creating a ground.  So the dangling oil pressure senor wire should not light the bulb but if the temperature senor is still connected, it might be the culprit (if there is no short). 

I misunderstood what your concern was.  I have used both copper/nylon tubing to oil pressure gauges with no leaks.  But I have black carpet and a drop or two would not show up.  I have read some people using AN type armored lines in fear of a line burst spraying hot oil.  To avoid oil inside the passenger area, you could use a electric gauge.  I like using both the light to attract attention and a gauge to diagnose what is going on.