I have a VDO pressure gauge and I have second one on my digital dash from EVODASH, both have their own sensors  plumbed into the same place as the low pressure warning. I get just over 4 bar when cold which drops a little bit (3 bar) when warm at idle but it goes back up when revved, to 4 bar again. Both gauges read the same (near enough). When driving they stay at 4 bar without much fluctuation. HTH?

A couple of thoughts.

1 oil level a bit high or windage plate missing in the sump leading to frothing of the oil so there is air in the oil and this limits oil pump as it is now pumping oil and air. Have you got the standard sump or has it been modified?

2 oil not draining back to the sump and sitting on top of the cylinder head. take cover off and check that drain holes are clear.