Quote:
Let me see if I understand, you are saying that if the oil is too thick, then the oil pressure could drop so low at idle that the oil pressure warning light comes on, but the oil pressure will rise when the revs rise & the light goes out, ....... with oil that is too thick.

Now I'm always up for learning new stuff, so perhaps you could explain the physics of how that could come to pass, because the logic of it completely defeats my tiny brain.

Looking forward to your simple yet comprehensive explanation.

I had a situation a while back where the 1200 engine in my wife's car was doing the same thing. I had reconditioned the engine within the previous 6 months & everything in it was new. There were no undue noises from the engine & it ran as sweet as you could wish.
I replaced the pressure switch & then the oil pump with new.

Eventually I switched a spare engine into the car & steadily dismantled the 1200 engine to find the cause.
I didn't need to look far as I found something strange in the sump. It was the rearmost camshaft bearing. These bearings had been replaced during the recondition job & the rear one must have been loose in the bore. It was presumably pushed out with the oil pressure from the welch plug side. Every time the rearmost cam lobe went round it reformed the bearing to lie flat against the back of the block until the whole thing eventually fell right out.
We put quite a few thousand miles on it with the last part of the camshaft completely unsupported at the rear end.

It was replaced under warranty & the engine was reassembled & reinstalled to give many years of reliable service after that.

The only symptom was the red light at idle, .... now that's Datsun tough.