Myarmar,
I performed the test as you have outlined and all the gauges behave as you have described. The resistance of the outside temp gauge always reads 59 Ohms.
So I am assuming there is nothing wrong with the gauges, so it is either the oil pressure sender or the power supply to it.
Then it dawned on me that I may have been missing with a sender other than that of the oil pressure!!
Please tell me where would the oil pressure sender be on a V12 engine?

The one I have been looking at is immediately behind the oil filter in the direction of the driver.

Thank you.