1968 Barracuda 383/4spd

I have an oil pressure indication issue.
Mechanical test confirms oil pressure. Done.
Ground grey wire unhooked from sender and turn key on, gauge sweeps smoothly to high then fluctuates, I assume the fluctuation means that the voltage reg is working.
Multimeter at terminal end, key on, unhooked from sender, moves to 4-5v, again, fluctuating.
Sender (for dash gauge, not idiot light) grounds fine to the engine and body with a continuity test.
Sender (new) ohms test shows 19-22 at idle.
But put it all together and dash gauge doesn't move. All other gauges operate, temp, fuel and amp. All operating.
What am I missing?