Get a single pole double throw oil pressure switch. Use common for ground and
have both idiot light and hobbes constantly powered but seeking ground. Go
into the switch "normally closed" with oil pressure, and "normally open" with
the hobbes. Attach "common" to airframe ground (-12vdc)

When the switch is static (i.e. no pressure) normally closed will conduct
to common, thus grounding the idiot light, turning it on. Vice versa for the
hobbes when the switch gets pressure, normally open will then conduct,
which will apply ground to the hobbes.

The one in the link I posted previously in fact does that I believe (has
a COM with, NC/NO contacts)

Regards,
Jeff Hays