Benzmac or MBDOC has posted that they have to flush out the cooling system repeatedly with a citric solution to get rid of all the traces of oil.
I think it was Donnie that made the post. He said it's real hard to get rid of all of the residue.
Hopefully, that's your problem...
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".