If it was grounded out, it would read 0 or 3bar, with no fluctuation at all. If it goes down a bit at idle then the pressure sensor is there, and working.

M103 engines can leak all over the place. Front timing cover is the usual suspect, along with the rear main seal, valve cover. etc.

Do you ever find a puddle of oil when its parked for a while?

If not, you are probably burning it via your valve stem guides.

__________________
Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2