It's difficult to say what's wrong with your car but my guess is that a different brand or viscosity of oil will not help. I suspect there is a bad pressure relief valve or loose bearing or something like that causing you to lose oil pressure. Are you sure you're hearing a valve and not rod knock?
My advice is to start a new thread and title it more specifically like "M103 oil pressure problem." Maybe that will catch the attention of those on this list intimately familiar with your engine. To take nothing away from the excellent advice you've received thus far, of course.
My M103 oil pressure can be as low as .3-.5 bar when idling in drive. I sure hope it's not developing the same problem.