loubapache is absolutely right. I have an 87 300SDL (255k mi)and a 98 VW Jetta TDI (195k miles), both of which have catalytic converters.
I bought the TDI new and put several thousand miles on the 300SDL. All along, I've used a mix of the white bottle Power Service or Howe's Conditioner and Amsoil Cetane Boost with every tank. The idle on both cars is smoother, start better, and have less smoke compared to not using the additives. If I'm on a road trip and do not have the additive, I can tell the difference by about 1/2 way through the tank without additives. Also, I see a slight increase in MPG (more noticeable in the MB than in the TDI).
BTW: If you are concerned about the fuel system, I would recommend the use of additives. Maybe not at every fill up, but often enough. My TDI has the original pump and injectors. The 300SDL has the original pump and the injectors lasted about 254k miles. I had one injector get noisy, so I went ahead and replaced the set. The owner's manual of the car specifies not to use additives due to the trap oxider catalyst. Once it is removed and a regular converter installed downstream, as long as the additive doesn't contain metals, you'll be fine.
Everyone I know locally uses either Power Service, Redline (not the diesel fuel catalyst), Howe's, or Amsoil. The reason I use Power Service is because it is widely available, inexpensive, and turns acceptable results. Amsoil is about the only company I know of that offers a commercially available Cetane Boost product. It is sometimes a hassle to have to mail order Amsoil products and they are a bit pricey. A while back I purchased several individual cans and a 5 gallon pail of the stuff! As the cans get empty, I just keep filling them up from the pail...
If you'd like any more information about diesel additives, email me at LDA4793@UDNET.NET
Sold: 1987 MB 300SDL, 320k miles, Astral Silver, Blue Leather, all books and records from day one.
Totalled: 1998 VW Jetta TDI, 488k miles, Classic Green/Gray Leather, loaded, E-Codes, "Slightly" modded, "Slightly" faster than stock, arrived 11/07/1997.
MBCA Member, Chicago Region