Correct me if I'm wrong on this folks, but aren't the oil-injection pumps used on most mopeds the 'impulse' type. They don't hold any nominal, steady pressure, but just give a single shot on every revolution of the engine.. ?
My point is, I'm not sure how much a gauge would tell you.
I believe that most of the idiot-light sensors for cars come on at very low pressures, like 3-5psi. Maybe there is enough pressure to make the output from that type of sensor useful.