Oil pressure gauge issue - variable with electric load?

[aero four]

Recently purchased an 83 944 and have noticed some eccentricities in the oil pressure gauge. Today it was 70 degrees, took it out for a drive. Engine on, pressure goes to 4 bars, and 5 bars as I accelerate. After about 15 minutes of driving, the idle pressure goes to between 2 and 1.5 bars.

When I turn the lights on, the pressure gauge drops a full bar, to as low as 0.5, and occasionally drops to zero if I turn a few more accessories on. In addition it sort of has a tendency to flicker a bit. The oil pressure idiot light has never once come on, but shines with key on, ignition off.

Does this sound like a faulty sender or wires to the sender like I think? I haven't the iron stomach to do a bearing job on this car. I am changing the oil to 20w50 this afternoon to see if that changes anything.

[Slam]

Grounding problems are endemic with these cars at this age. You might want to spend an afternoon in the garage hunting down the grounding points and sanding the connectors. Look for brown wires. There's a ring of them under the dash behind the fuse block, these corrode. Since the dash wiring is in series, bad grounding can and does cause what you're describing. I usually go so far as to remove the dash and fab up a new ground harness from the back of the battery box as a redundancy measure. If you've got the Haynes manual it'll tell you where the major ground points are. Off the top of my head, look just behind the headlight buckets on the "frame" rails there, the aforementioned ring under the dash, the rear of the car near or between the tail lights and anywhere else you find a brown wire skulking. Of course, a biggie is the battery ground to the bellhousing. In many cases replacing this cable is the best way to go. I added a strap from the battery to the intake manifold too.

Have fun!