Oil pressure dilemma in V12

[Anthony]

Earlier this year I rebuilt my V12 engine and afterwards the oil pressure was good at 60/70 psi.
Recently I changed the oil after running the newly built engine for a little while. I replaced this running in oil with Halford's Classic 20/50 but stupidly I didn't change the filter.
Now when I start the engine the oil pressure gauge immediately rises to 40/50 psi and then almost immediately swings back to zero and stays there regardless of revs.
There are no engine noises but for obvious reasons I don't want to run the engine again until I discover and correct the problem.
I have read the multitude of posts on this subject but I have limited time and was hoping that someone could give me an idea of a sequence in which to check the potential offending components please. I know the sender and gauge are notoriously problematic but I can't afford to keep replacing parts hoping it cures the problem and then finding they don't.
Advice would be most welcome please.

[cactusman]

I would start by borrowing a mechanical gauge and then connecting it to the engine and cranking without starting...pull off one low tension lead from the coil...check the gauge rises and remains above zero....if no oil pressure then either there is an issue with the oil pump or the filter/pressure release...probably. if the pressure rises and holds the issue lies with the gauge, pressure sensor, voltage regulator for the gauge or wiring...probably....FWIW I'd suspect a measurement issue and not an engine issue....

[AussieEtype]

I would agree with Julian - the fact that pressure does come up indicates to me a measurement issue. You haven't mentioned what is happening with the oil pressure light - if it is staying off then definitely a gauge issue.

As an aside - you have a new engine why have to you chosen to use 20/50 oil rather than something like 20/40? If the engine was getting on a bit I could understand but I would not have thought that a heavier oil would not be needed on a new engine especially in colder climates.

Just curious thats all

Garry

[Adamski]

Don't worry as if your red no pressure light hasn't tripped then you should be ok as it's from another switch.
I had this and solved it by this previous posting. There are fake sender units out there but this fixed it. Early cars like mine a pig to get to though.
I changed supplier to David Manners and received one that fitted and performed perfectly. Made in England. Spade terminal was to one side but wire was long enough. Too many bad repro items out there but this supplier is very good.