I've been told by a very good Ford Fleet Service manager that running anything other then ultra-low sulfur diesel in the 6.4 WILL not might but WILL clog the emissions system. He also went onto say run wmo in a mechanical inj. system. The debate rages on about running in a PS 7.3 most say no in a 6.0 and in a 6.4 no way. Do what you want but in anything newer then a 7.3 no way would I do it.
By the way. I do run it some in my 7.3.