Here's what we have: Non original speedo, it is a VW Speedo with
working Idiot lights, just not original for the year. Original wiring
harness is intact, correct wiring for the idiot light and pressure
sensor in the block. I had to run 2 wires to the rear, one for the
pressure side of the switch and one for an oil temp sensor in the case.

I removed the aftermarket sensor and plugged in my orig. pressure
sensor. Everything works as it should, idiot light on with key and
turns off when it cranks. So...there's no problem there. The problem
must be with the aftermarket OP sensor. I removed a rubber plug to look
inside the sensor. Everything looks like it should. Copper contacts are
in 'closed' position. The only thing I can come up with is the size of
the oil pressure 'hole' in the sensor. the aftermarket hole (where oil
enters the sensor) is considerably smaller that the original VW sensor.
Prehaps if all else fails i can drill it out to a similar size. I can't
see how it would make a difference as long as oil is entering the
sensor it should be the same pressure. So.. thats where I stand. If
anyone know of a different 'kind' of OP sensor, that is, if there are
indeed sensors that work mechanically different, please drop a line
here. I've never heard of such a thing but that's not saying much!
Thanks.