Ok, I have tried to disconnect the wire, and the gauge went to zero. I will try to ground it too. I found a sender at Banks, I think it's a bit expensive, is this the one I should buy?
The car have run a long distance like this, I think. I don't think the PO was aware of this, he thought the oil pressure actually went up. I have driven 360 km and not too easy, I think. I am wondering how healthy the engine is.
Regards