Mouss,
The pressure relief valves are adjustable but I doubt that is the cause .
My guess is it could be wear/excessive clearance on the crankshaft bearings/journals and /or Camshaft bushes .
I doubt the problem would be with the oil filter and oil cooler.
I would initially try a thicker oil to see if it helps .
Penrite have a HPR50 which is a SAE 40/70 and designed for older engines,with larger clearances than modern engines.
At the worst it will need a new eccentric rotor oil pump and/or complete engine rebuild.
Definitely check the pressure on another gauge , as many older gauges are not accurate.
You or the owner should get advice from a Pre WW2 BMW Specialist , there are surely some in France .
Regards
Geoff
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