OK, finished putting in my ghetto oil pressure gauge setup from misc. pieces and parts I had sitting around at work. I used a "T" to retain the stock oil pressure switch.
Here's the question:
When does the stock oil pressure light "light" up?
Does it light when the ignition is initially turned on? When the starter is cranked? Never? I can't remember now that I have modified the system and I want to make sure the stock idiot light still works as a backup. I would assume that if I had screwed up the stock sensor, it would be constantly lit, but I need some confirmation.

Mine comes on if I turned the car on, but not start it. If you are driving and, the light comes on, its a time to buy a new motor light. I droped 3qts of Mobil 1's finest on the Texas interstate, and my light didn't come on. Normally oil pressure is ~50psi at 3000rpm. I was seeing less than 20psi when I noticed it.

Is that in the '91 Classic or in one of the 200SX's?
I'm wondering if maybe they (classic vs. 200) are different?
I was really surprised that the light does not come on now. I probably screwed it up. :)

FYI I'm seeing mainly around 55psi in normal driving conditions, dropping to 15-20psi at idle. I'm hitting close to 80psi under wot.

That was in the 200SX. I'll check the '91 when I leave for home tonight.

Like Eric said, if your oil press light comes on, its already too late. Mine never even came on when my old engine went bad. I am just using an autometer oil pressure gauge, and I totally disconnected the dash light.

Maybe I was a little unclear.
I have connected both my new gauge as well as the original OEM switch via a Tee fitting. The gauge works fine, but I am questioning whether the idiot light is still working. I retained the idiot light as a backup as well as an indicator for at night as my ghetto pressure gauge is not back lit. I'm just curious if I have done something to the stock unit as it does not light up when the ignition is turned on and I can't remember if it did previous to my banana slinging installation. Let me know what you find Eric. Thanks in advance.

When the key is in the ON position, the oil light should stay on. Once you start the car, it goes off.

I've been told that the stock oil pressure sender will throw a light after it's too late. Something like 4psi or something like that LOL.

I just removed the stock sender when I installed the autometer sender. I also didn't want alot of extra weight hanging out of the oil sender hole so I just kept it simple with one sender.