High Oil Pressure Buzzer

[Charles Schwartz <cschwart@stern.nyu.edu>]

Hi,

I have an '86 4KS with 114k on the 1.8.  I usually run Castrol GTX 20W-50
(with Mann filters) in the warmer months, and I didn't get a chance to
change the oil until recently - on a VERY COLD day.  

On my way out to East Hampton from NYC (with the 20W50 at about 5F), about
a 2 hour trip, the oil pressure was pegged all the way to the right.  The
buzzer went off several times.  I slowed down to about 45mph and then it
would shut up.

I changed to Castrol GTX 10W-40, and the oil pressure dropped when it
warmed up to normal readings - just under or at 5 at 3000 rpm.  The buzzer
was silent.

Yesterday it was in the 50's and I drove upstate to visit my sister.  The
car was fully warmed up, the oil was full, the pressure gauge read normal.
The buzzer went off sometimes.

Could this be an oil pressure sending unit?  A stuck pressure relief valve?
 Something worse?

Thanks,

Charles