I run an Edge insight multigauge (it plugs right into the OBDII) and two of the parameters I monitor are Coolant Temp and Diesel Engine oil temp. If you get more than a 25 degree split - then you have oil cooler problems, and EGR cooler problems will be on the way.
I'm going to get two more gauges - HPOP pressure and fuel pressure, because those are 2 more areas where monitoring can pay off - HPOP leaks, and low fuel pressure.
I wouldn't add any simple green or any other snake oil, get a professional flush, with the recommended coolant and mix (no tap water) then install a coolant filter- any of our sponsors will carry them. They are not too expensive, and are pretty easy to install. My 06 had alittle casting sand in the filter first time I checked it, but I have a feeling that the early 6.0's had more casting sand issues than the later ones.