Oil Pressure Gauge Reading

Hi,

I have a 1995 E220 and the oil pressure guage
always reads 3.0 ( The maximum). I know,
and have driven other cars, including other
E series, and the pressure guage always reads
low with engine at Idle speed & icreases
as the revs go up.
What could be the problem? Obviously a
faulty guage is a possibility. What other
Factors could be taken into consideration?

Thanks

Mehdi