When I was looking at an '87 300SDL the issue came up, and I learned the following at the time (more experienced members please correct me). The material in the trap oxidizer gradually deteriorates over time and the fragments can be sent through the turbo, hence the occasional need for the turbo replacement. After a time, the same process begins with the replacement TO, and MB suffered through repeated replacements on a number of cars. A couple of years ago MB came up with a replacement for the whole system, the oxidation catalyst (sp?), which ostensibly solves the problem for good. You should call customer service at MBUSA; if you give them your VIN they will confirm that the OC recall has not been done to your car and authorize a replacement (they were ready to do this on the '87 I was looking at but the PO had replaced the trap oxidizer with a straight pipe, invalidating it from MBUSA perspective).

Check the following link for 350 engine issues:

http://www.mbz.org/info/complaints/350D/