Jari:

When you first start up the car, oil pressure is usually low, and then should increase with RPMs. My 1981 280 SE, W126, with a 110 engine, hovers at 1 Bar at idle, then goes above 3 Bar at anything above 2,000 RPM. This is good territory, and you should experience the same.

If you have low oil pressure above 2,000 or 3,000 RPM, then you should have your oil pump checked. Anything lower than what I've stated will not put enough oil onto the journals, etc......Andras