Whether or not it's approved or recommended by mb 229.whatever, if its not ch-4, its not an acceptable oil for diesels. You are probably correct in your statement about the 15-40 being acceptable for gassers.
As far as the tdi goes, most experts agree, even the 10,000 mile oil change isn't a very good measure of the TDI engine. Most would agree the first oil analysis that will be worthwhile for data purposes would be the 20,000 mile oil change (the 10k between 10 and 20k). VW's oil change intervals are 5,10,20,30 and so on. The 5 and 10 are simply skewed by the break in wear metals, and the change in oil type etc. THe best judges would be a 20 and 30k samples. I'm not saying synthetic rotella t is a bad oil by any means, just that at 10k its not a good measure of the engine or oil's performance. Especially since the person sampled at 5k and the recommended interval that will be followed most likely is 10k changes.
2004 Ram 2500 Cummins HO
2000 Jetta TDI
1999 E300 (sold)