sounds like you may have a voltage regulator problem, it is common
with the C4's i think they upgraded the alternator in 86 or 87 so if
you do have to change out the whole alternator i would get the newer
one.

80 psi o the oil pressure all the time is not right at all, i would
start with a new sender from GM or you can take it to a shop that you
trust and have them hook a gauge tester up to it, most shops that do
much electrical work will have one of these little units, you might
try unplugging the wire from the oil pressure sender and see if your
gauge goes to 0 psi. i believe that (from memory) that most GM's go
from about 1 ohm at 0psi to 80 ohms at full gauge deflection, i am not
sure about the digital dash like yours has, but that is the way most
of the GM stuff in the 80's worked.

as far as what is normal, that depends on a lot of factors, bearing
clearances, temps, (above 180) oil viscosity, rpm's. But a general
rule of thumb is ten psi/thousand rpm with a fully warmed engine
MINAMIM.

on my 87 at about 1500 rpm i see about 37 psi motor hot and running
Mobil one 0w40.
Greg

Get your voltage/charging  problem solved before you work on your oil
pressure reading.

Bones
'87 & '95

On my 85 when idling I am around 55 psi for oil pressure and its up to
around 70 when toying around. I use mobil one 15 / 50.

On your problem... are you sure that the compressor is not seizing up and
dragging down the rpms? Does the serpentine belt squeal? If you were
suddenly not charging the battery should keep you running.

I have 129k on engine although not sure if it ever been apart. In general
does a 350 push oil at this milage? Passed emissions fine here in Mass at
around 20 to 50 percent of allowed emissions. I assume that past owners were
not babying it as its red and 4 plus 3.