my engine did the same thing oil light took a while to go out. I did up the oil pump (parts still available from veale) including relief spring. helped a bit put a new sender on still the same. then i cranked the idle up to 900-1000 rpm oil light goes out pretty quick now a pretty dodgy thing to do but ive done 15000km since no worries and now ive turboed the engine and still havent had any troubles

Time for a ca18det conversion

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Why did you replace the last oil pressure light switch? Same reason/symptom as now?

If it's not a dodgy new switch, I'm picking muck in the pressure relief valve keeping it from completely closing at low flow/idle (given these are exposed to unfiltered oil).

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I've had a few cars with very high mileage do this with a dying oil pump, and have found that thick oil can actually help the problem... It will take a little longer to bring pressure up when you start the car, or you can give it a small blip on the throttle to get the pressure up, but the thicker oil hangs up in the engine better so the light doesn't come back on at idle once the pressure is up the first time. I generally use 40W70 for this kind of purpose (yes it's pretty hot here in Adelaide, so you probably don't want to try this in the winter in the US, or if you live in Thredbo or something!)

If it's an engine that you want to keep long term though, just get it checked out and fixed!

good luck, for me it's never one of those simple things.

yes the light stays on longer than normal when starting. im driving it on thursday, so i will see if it does it again, and if yes, i will print this off, and go talk 2 the mechanic. i think its a bet 2 much 4 to try fix

Nah, my money says that it's a little bit of something that is preventing full closure of the oil pump relief valve. At low engine speeds the pump does not provide much volume or pressure & it doesn't take much to bleed off enough extra pressure to bring it down into the 'light on' zone.
As revs pick up, the volume & pressure are enough to overcome the losses at this point.

Well, that's my theory anyway.

As for that back bearing thing in my engine, I suspect that it's a once in a lifetime problem.

haha yeh it is Datsun tough. Becuase when that happens to the commy V6 they snap a camshaft because it seizes the far end of the shaft while the chain keeps driving the other half.

Lets hope its not that severe this time!

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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.

This will also happen if your bearings are worn. Particularly if you've given it a hard time or driven it on low oil. Hope its not but it is a possibility!!