One important thing to note is that the temperature really isn't that big of a deal, so long as you don't have any uneven heating going on (such as what happens when you boil over, and steam pockets form inside the engine.)
I was running the stock Motorcraft fan clutch, less that 1 year old, looked to be in perfect shape (bought it N.O.S. off ebay in it's original box) I got my temp pretty high on a long tough pull, fan clutch never kicked in. Temp light never came on, but by the time I got down the other side of the hill, I had developed a nasty bottom-end knock.
Why ? You ask ? Oil viscosity... As oil heats up, it looses a great deal of viscosity. It becomes runny and thin and if it gets hot enough, it cokes off and breaks down. I believe that is what happened to my truck. I had just changed the oil before the trip to Valvoline premium blue (with the Cummins seal of approval on the bottle, should've known better...)
My oil pressure when I reached the top of the hill, at 3,000 RPM was down to less than 25 PSI !!!
Even though your cooling system might be able to keep up, and it's true that diesels do run more efficiently at higher temperatures, the biggest thing in my opinion is oil temperature.
My black truck has a locked clutch, and I am never going back...
