I just installed a new gm crate motor in my 91z28 350 tpi and noticed that the oil pressure reads about 30-40 on startup and then goes down to 0 after about 10 minutes of driving. I have not hooked up a manual gauge to it yet. where is the easist place to do this. some say the oil pressure switch( brand new) but if that is taken out then will the fuel pump turn on. dont they work together.

 

I had the same problem after i had my engine swapped. It was indeed the oil pressure switch (which was leaking). After i got that fixed the gauge was peaked straight up because the sending unit was bad. Got it replaced and all is well. good luck