Just run 12V to a small light under the dash and ground it through the
original pressure switch.  It will light when there is low pressure and go
out when the switch opens, which is around 5 psi.   The other option would
be to get an electronic oil pressure guage.  Some come with a warning light
switch built in.  I have a JC Whitney gauge that reads 0-100 psi and has a
switch that opens at around 10 psi.  I ran 12v to the gauge and mounted a
light next to it.  The sender has one wire for the guage and one for the
light.  Works OK, although a VDO would be much more accurate.  I still have
the original light in the dash.

                                                            Jay