WVZR-1

This:

http://www.bwdbrand.com/online%20%20...g/content.aspx

The complete link won't copy but just use the search block and add your S4048 and see the results. Click on part # at top and enter part number to the left!

Then look at an S4087 - still cheap at $32 or so $$

C4ProjectCar

Sorry for not replying for so long - been busy and then went camping.
You're right. That's the switch for the fuel pump to shut it off if pressure goes too low. Thank you, O'Reilly's, for selling me the wrong part. That's strike two.
I heard that I should replace it with an AC Delco unit because others are unreliable. Is there any merit to this?

WVZR-1

I wouldn't say so. It might depend on your relationship with a GM parts guy or an ACD jobber. These days it's very difficult to sort good, better, best - crap shoot at best for almost all of them.

C4ProjectCar

Thanks.
I still haven't gotten around to getting a mechanical gauge to test the pressure because I've been fixing other problems, but I have noticed one thing in the process. Once the oil pressure gauge starts to drop, the needle kind of jerks and shakes back and forth occasionally, like a twitch. Is this a clear sign of anything?
Also, no matter how low the needle goes, the low oil pressure light never comes on. I looked, but I couldn't find a light that said "low oil pressure", so I'm assuming it's the one that just says "low oil"?
As I understand it, that should come on at 4 PSI, and I was clearly below that threshold.