Hi Trevor @trevorkent
I’ve had this jam on two of my XJS. The first freed itself by me pushing the reset button intermittently as I drove along. The other I had to take the console to pieces and free the jam – I then left out the reset button so it couldn’t happen again.
The Oil pressure gauge issue sounds like a loose wire, rather than a faulty gauge to me – though I’m sure there are others on this site who could give some good advice.