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Last evening a weird issue happened. As I was driving (in city traffic, low speeds) suddenly I saw VERY LOW oil pressure numbers on the digital instrument. )
Numbers such as 0.3-0.4, normally meaning that I should stop the car immediately. However, the dash oil light was not on (before summer, when that issue with my oil pump had happened, I remember that the dash oil light was coming alive whenever the digital gauge was falling lower than 1.0). Also the sound of the engine was absolutely normal.
So I drove home (was lucky enough to be already quite close to my parking lot), and turned the engine off. Checked the oil, was not too low, however I added some more. I turned the engine on again some 10 minutes later. Everything ok. Today I drove about 4-5 km, everything normal.

Could this be some burr, some tiny small metal part, obstructing the instrument, misleading it to give wrong figures?
As we recently replaced the piston rings, in order to reduce too much oil fumes, The engine was opened, so I am in the period of having to do about 1000 km before changing oil and setting up valves (I haven't done more than 600 km yet from that point, thus oil has not been changed after engine opening)

Sounds like the actual oil pressure was ok. I had something slightly similar with my kit, oil pressure would only rise very slowly after startup even though the warning light turned off immediately. This issue went away after some time, felt like the sensor was obstructed or clogged temporarily. Strange, but if something like gas tape was used for the sensors in the sandwich plate this can perhaps be a cause as well.

That will need some investigation. I really hate problems like that. Good luck solving it.

Well, I forgot to mention something else weird-but I don't think that this has anythung to do anyway
When I opened the hood, the throttle cable nuts were loose (two nuts that hold the cable tight on the webers)
I tightened this also, but as I said, I guess that has nothing to do with the oil pressure indication
I think I will start looking into the matter, only in case it happens again after oil change (which will be done in abt 350-400km)

dont see why you shouldnt change your engine oil as a precautionary measure at least! also I always change the oil regularly after an engine rebuild just to make sure any possible metal flakes etc caught up in the engine during the rebuild is immediately cleared out of the system as soon as possible ( first engine oil and filter replacment after a few engine runs, than another after a few 100's of km's then the next after a few 1000's km then just regular interval changes)

Changing oil? Not a bad idea at all
I will probably do this in the next few days.

Already mentioned the issue to Peter (who manufactured the gauges)

Today took the car at work, and will do some km later in the day
No problem at all so far

Wondering if you'll find any dirt in your oil.

When I checked it the other night it looked very clean however
(only 600 km)

Spoke with the mechanic
He says he does not believe it is a metal rubbish or anything else with the oil, he thinks this has to do with the instrument.
Will see...

As I wrote, I had something slightly similar and for me the pressure sensor is suspect no 1. But since the issue went away and the warning light stayed off I wouldn't worry too much.

Yes, I do not worry too much as well, it's just that I would like to have a trusty instrument and not a red blinking number