On most older Bridgeport, the oiler runs only when the spindle is on.
It runs a cam that lifts the plunger. This takes about 20 minutes. The lever for the plunger falls off the edge of the cam and pumps oil. It is spring loaded.
Yes, there are metering units that hold pressure and thus oil flows at a predetermined volume though each one. If the metering units were not there the oil would go through the path of least resistance and the rest of the machine would get no oil.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)