Only Plug #7 Fouling After 300 mi

My sister owns a 96 Yukon 5.7L AT into which a remanufactured engine was dropped. The motor was done professionally, but the swap was done by her ex. There are about 2000 miles on the new motor, and it has consistently fouled the #7 spark plug every few hundred miles with build-up resembling the 6th picture at http://www.theultralightplace.com/sparkplugs.htm (http://www.theultralightplace.com/sparkplugs.htm)

She says that it idles a little rough, the truck blows no smoke, and #5 plug looks like it's got one foot out the door to going bad. All other spark plugs are normal. I am outside the country, so i cannot check it out myself, so without seeing it, i am asking for ya'lls GMC specific expertise. Now, the company that rebuilt the motor says it has no reason to believe that the engine is faulty, and her friends think it has to do with a fuel injection problem...but i don't know how mechanically inclined her friends are...

With just a single spark plug (maybe two) getting fouled, can the problem be injector cleaner and/or the wrong oil like theultralightplace.com suggests? Any and all ideas and advice are welcome. Thanks ya'll.

Welcome to AF. Oil fouled or fuel fouled, that is question here? That pic can be from either.