I think I've opened up a can of worms.
ANY warning device is sort of like using a radar detector. For instant-on and some lazer, you are pretty much toast the moment you hear a tone.
An auto-shut down (as Blas is suggesting) is great if you can discern that it was low
oil pressure that triggered the event and not a short circuit to the ignition (been there, done that...flat bed tow home just to learn I had a loose ground wire on my MSD box). I still grade myself as "suck" when it comes down to electrical trouble-shooting (but I can hang with spinning a wrench with most of you).
I'm trying to keep this simple. The toggle switch method with a $6 buzzer is K.I.S.S, light weight and easy to mount/hide. Adding a on-delay relay adds some minor costs and is more a task of just sourcing the right component (Would Fry's carry something like the Amperite 12D2-20SST1A that was suggested?). I can also handle a soldering gun and planned to do that with all connectors anyway. Auto-shutdown would involve wiring some sort of ignition kill device and that's out of my comfort zone. And I don't outsource most of my work...I'm a lean Cobra owner.
I do have quick access to a local Radio Shack and weekend access to either Fry's or another really good local place in Fullerton that is an electrical supply house called Orvac's. So those that "build" these things, please school me on what I need to acquire. It's becoming a good winter project while we get more waves of rain.
Thanks,
-Dean