4000 Oil Pressure Questions

[Dustin Wolpoff <melon@centuryinter.net>]

Hi, my 1986 4000csq with the 5 cyl  and 160k recently started giving me
the oil pressure buzzer warning at about 2500 RPM and up after driving
for 20 minutes or so.  I put a can of STP in it and the problem went
away for about another 20 minutes but then came back.  Do I need to be
concerned?  Is it a pressure problem or a sending unit problem?  If I
put a manual gauge on the engine, what is a good reading and what is a
bad reading?  Thanks in advance.
Dustin