maineiac - got it. Right, I googled all those issues on EngineBuilder.com As far as I know it has not been bulletproofed. As I said in OP, 2 injectors are sticky @ 113,000 miles so all 8 are being replaced now. Supposedly the truck was not used for heavy load / often so he's gambling on the head gaskets. Not sure about the cooled EGR.
He's 20, an engineering apprentice, with 5 years engine overhaul experience. He's also a poor college student taking gambles with what he can afford, so I just consult. The job at hand is what modern motor oil best fits this thing in our ambient conditions in mid-Minnesota.
I googled the original O&M. API CJ oil hadn't even come out then (2003 manual). Recommended vis was 10W-30 "normal usage", only covered -10F to +90F. It routinely gets below -10F and above +90F here. I know, who the [censored] would live here, right? 40 years ago there were great jobs here.
I've googled the current Ford Diesel Filter & Oil Reference Sheet, published Feb. 2019. They revised 10W-30 "normal usage" range from 0F to 100F+, 15W-40 (mineral) from 20F to 100F+, and added 5W-40 (synthetic only, to my knowledge) from -20F to 100F+. They didn't include their tirade about having to meet Ford wss-m2c171-f1, publishing that elsewhere, later.
Ford doesn't seem to really know what they're doing. Navistar may have been convenient, but that whole joint venture was a mess. It is what it is. I haven't seen a reply yet to the original question which of the 3 approved synthetic 5W-40's is preferred. I appreciate all the history, but it's not really the point.