Black/blue should be hot when the lights are on. Parking lights or headlights. Gray gets plugged into the black/blue so that it also gets power when the lights are on.
Green/black is the oil temperature sender wire. The sender is a variable resistor, the "signal" is the amount of resistance from the wire to ground.
Check this recent thread on the same subject:
http://forums.pelicanparts.com/porsche-914-914-6-technical-forum/667878-few-loose-wires.html
--DD