Constantine, a member of the MBZ diesel email list, just installed a factory crate motor into his 350SDL after the bottom end took a dump. He's in the process of starting up the new motor for the first time as I type this. Add that to the list of people with blowed-up 350's!
As to the exact cause, who knows. I don't buy the carbon chunk theory one bit. I do agree with disabling the EGR and ARV, if your engine is so equipped, but this is a LOT harder on 1990-up engines.
About the Slick-50. I don't mean to sound offensive, but you are a fool to continue using that junk. I used to use it myself, and the last time I put some in any of my cars wa back around 1997. (I used it from about 1990-97, and only still own one of those treated cars now.) Then I learned the truth about Slick-50, and other oil additives. It's absolutely a terrible idea to use it in ANY internal combustion engine. I'm annoyed that Slick-50 (and other, similar products) are allowed to be sold at all. PLEASE do some research about it before you use it again. PLEASE. After much personal research on the topic, I strongly recommend using a top-quality, Group IV/V synthetic oil such as Mobil-1, Delvac-1, Amsoil, or Red Line... and not using ANY oil additive at all, none, zero, zip, zilch, nada.
Some links:
http://skepdic.com/slick50.html
http://www.thegsresources.com/garage/gs_additive.htm
http://www.smartsynthetics.com/articles/facts_about_aftermarket_oil_addi.htm
Best wishes,