The gauges are notoriously inaccurate and so don't rely on the information you get from them. If the oil light goes out that's good as it is on a separate system, the gauge (assuming the info is good) should read around 4-5 bar. My engine was completely rebuilt and the gauge only ever shows 3 max. At tick over you may not even get a reading.
After putting my x19 back together I found that the oil pressure gauge dropped off when it was ticking over, and I panicked and switched off the engine! I checked the oil pressure by disconnecting the HT coil lead and cranked the engine over and the red light went out ok! After this I ordered a new oil pressure and gauge switch from Claire at Eurosport and fitted both at the same time as the originals were probably 30 years old !
I also found that the dashboard must have a loose or bad connection as the gauge stopped registering and I had to tap the dashboard to make it work?