oil temperature gauge operation?

[Roa, Greg]

Hello all,
When driving the 86 4kq on longer trips, I've noticed that my oil
temperature stays at a reasonable reading on the console gauge, except when
I have my headlights on.  When the headlights are on, often the gauge will
climb up during the trip.  However, when I turn the headlights off, the
gauge seems to drop several notches within a few seconds.  The only clue
that I have is that the voltage gauge often drops from the 13.5v mark to the
13v mark when I turn the headlights on.  Not always, but often.  So, could
this be a bad ground or alternator time, or just a strange coincidence?  I
would have thought that the oil pressure gauge would read lower if the
voltage was lower, so it doesn't make much sense to me...  Any clues?
Thanks,

Greg Roa
Cincinnati, OH
86' 4kcsq
93' 90 CS
83' 944