Does anyone know the thread size for the oil pressure switch?

Is it 1/8-27 NPT or 1/8-28 BSP?

Are the oil switches normally open or normally closed?

The parts catalogs are not very consistent ... but I found:

Beck/Arnley Engine Oil Pressure Switch 201-0031

I need a 1/8-27 normally open, closing @ 7-15 psi

Not sure about the thread size but the switch is normally open and I think closes at 8 psi

What model car?

I just looked at a spare I have and it seems to be a M14 x 1.5 thread? 1995 XJ6

X308

... but it is to fit a replacement warning switch after extending to a tee-block which has 1/8-27 NPT female thread.

Finding a normally open until a pressure > 5psi is like hen's teeth. Any pressure you want for normally closed, but not the other way around.

..umm in the list of cars for the senders, the one that sticks out is XJS or the V12 XJ.

by the above, you mean that below circa 8psi, the switch is open? meaning that the circuit is not energised?

I know that's pretty much what you wrote ... the vendor catalogs are not very clear on what they are selling.