Omegabenz, with regards to your statement:

" I have heard from some mercedes mechanics, that every time they had a crank ground down, and they used undersized rod bearings with correct tollerances, and they still had low oil pressure."

Since millions of other engines have had this process done when they needed the crank journals trued... that is.. taking them down to the next size bearings available... and functioned with no problems afterwards....

What do you think/suggest/ suspect is different about the Mercedes cranks these mechanics who relayed that information to you dealt with which would cause oil pressure problems even when they followed normal mechanical procedures ?