A few points of clarification to Tom's post...
FSM allows the pressure of hot oil to drop to 0.5 bar at idle. A bar is roughly 1 atmosphere, which is 14.7 psi absolute pressure. So basically, the official lowest appropriate pressure to see on your gauge when you car is warm at idle is just above 7 psi. At any real load and non-idle conditions, you car should be pegged at max, which is probably 45 psi (aka approx 3 bar) on your gauge.

Sorry I cannot help with why you are seeing those low pressures. Does your car have an oil line that sends oil to the back of the gauge, or do you have an oil pressure sender which sends an electrical signal to the gauge cluster?

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).