Source: Lotus West
Title: LOW OIL PRESSURE SIGNAL
Here is an un-ignorable low oil pressure audible warning that you can make for $9 and one hour of your time. It is reliable and simple: an idiot light switch turns on a high-pitched signal device.
The switch is by Stewart-Warner and available at most parts stores, for $2. Type 364L activates below 6 psi, 364AH below 10 psi, and 364K below 15 psi. Pick one that won’t come on when your engine is idling on a hot day.
The signal is a Mallory Sonalert, available from Radio Products Sales in Los Angeles or your local Mallory distributor. Type SC628P emits two to five pulses per second of 2900 Hz sound, and at $10.70 is the most expensive of the Sonalerts. It is also the most un-ignorable. Slightly less scary is type SC628H, which puts out a steady 4500 Hz tone, and costs $6.85. Less money yet is SC628, which costs $5.90 and puts out a steady 2900 Hz signal. All of these Sonalerts have a sound intensity of 70db or more.
To install, tee the switch to the engine end of the oil pressure line and run a wire from it to one terminal of the Sonalert. On a negative ground car this will be the (-) terminal. Connect another length of wire to the other terminal of the Sonalert and tap onto the wire which runs from the red "ignition" or generator light (in the tach) to the voltage regulator. By picking up power for the Sonalert at this point it will operate only when the engine is running (and the oil pressure is down). If it operates immediately when you turn on the ignition, you have tapped the wire on the wrong side of the light bulb.
The conversion, and all parts mentioned above, is suitable for all cars whether pos. or neg. ground, 6 volt or 12 volt, generator or alternator.