I hope the new year is treating you well. I own a Falcon AU1 XR6 which I have had since new in March 1999. It has 200,000km on the odometer plus a bit. I change the engine oil and filter every 10,000 Km.
My oil pressure gauge has 7 increments from 0 to 7 and no matter how hot or cold, idling or driving revs, the engine oil pressure has always sat between 3 & 4 on the gauge. A couple of weeks ago I changed the engine oil and filter and bought Nulon oil for the first time. On my first drive of about 24km every time the engine idled the oil pressure dropped to just above the 2 mark, driving engine speed was fine.
I rang Nulon and they came up with some lame excuses like the engine is old or I need a heavier grade of oil. The pressure has never dropped like that in the whole I have had the car and I believe that I was just fobbed off.
Occam's razor is a powerful logic tool. You can bone up on Occam's razor here, but basically it says that the hypothesis with the fewest assumptions is best.
So, let's apply Occam's razor here: Your car is old but running fine. You take care of it by changing the oil + filter regularly, which has doubtless aided its longevity. You add Nulon, God knows why, and the oil pressure drops on idle. Conclusion? The lower at-idle pressure and the addition of Nulon are related.
Obvious confirmation: change the oil and filter again, but without Nulon, and normal at-idle oil pressure should resume. If that happens, you know it's a better idea not to use Nulon.
Oil additives are a con, mate. They don't achieve anything. Modern oil is already knee-deep in additives that are engineered-in to achieve the performance dictated by the specification to which the oil must conform. It's impossible to see how adding more would help, seeing as there's no defect in the modern oil, which needs to be remedied.
Ditch the Nulon, and go back to what you were doing before, especially since it's obviously not helping. This will also save you money.
The oil circulation system in your engine is designed carefully to pump oil against a known resistance caused by the oil's viscosity working against the clearances in bearings, etc., and the size of the oil passages in the engine. If you make the oil thinner the pump can't keep up at idle, when its volumetric delivery is minimised because the engine's running slowly. So the Nulon's not an asset, clearly. (The low pressure means it's running out through the bearings faster than it can be delivered by the pump, which is not ideal.)
On Nulon's advice: you don't need a heavier grade of oil. You simply need the right grade of oil without the Nulon in it. That's the truth of the matter. The manufacturer did plenty of tests to determine what oil was right for your engine. Frankly I wouldn't arbitrarily dick with that specification unless I had a really good reason to do so.
Since there wasn't a problem before (at least you never mentioned one) why try to cure one with Nulon? Don't get sucked in by the marketing. The whole additive thing is a scam. Regular oil changes are the way to go - and your maintenance regime is proof of that.