This is an ole one that I was thinking about and wondering if this may be the problem. My Dad used to tell his mechanics and me to stay away from Pennsylvania oil because it had a high ash content. It would reck havoc in a Detroit Diesel by clogging the injectors, they switched to Valvoline and the problem went away. This is really ole school stuff here.
http://www.baileycar.com/oil_overview_html.htm