So yesterday we were towing our camper home from a weekend trip. We were pulling on the interstate at about 70mph, up a slight grade into a strong headwind. The outside temp was about 85 degrees, so not too hot out. After about 5 miles I got antifreeze sprayed on my windshield, coming from under the left front corner of the hood. The water temp held at about 220 degrees where it usually runs when towing hard but the oil temperature steadily worked up to about 240 degrees. I limped to the nearest place to stop and pulled over to let it cool down. After it cooled down I added water to bring it back up to the minimum mark. We were 200 miles from home on a Sunday so I stopped in the local Wal Mart and picked up a can of stop leak and poured that in then we hit the road again. About 10 miles down the road the water again sprayed onto the windshield and the oil temp came up, this time hitting about 260. The water temp stayed between 218 and 220, once working up to 225, so I slowed down and the temps came down. I was able to creep along and made it 50 miles to a relatives house where I let it cool again and added more water. When we hit the road again we made it about 10 miles and the oil temp had worked back up to 240, water temp stayed around 218 so I stopped at my parents house and dropped the camper. We drove the 100 miles home from there without the camper. The water temp stayed around 195 and the oil temp held at 212. If I got over 60mph the oil temp came up, but keeping it under 60 the temp held.
So, what is wrong with my truck? Is the oil temp up because I was a gallon and a half low on water? The most I had to put in was a gallon and a half. Did I plug the oil cooler up with the stop leak?
The oil temp when towing has always ran 12 to 18 degrees higher than the water temp.
I forgot to add, 2005 F350 with the 6.0 diesel. 240,000 miles on the truck. I have owned it just under a year and don't know a lot about it. I don't know if it has had any work done to it prior to my owning it, the antifreeze looks orange or yellow colored.

 

Update:
Today I drained the radiator. The first little bit that came out was golden yellow colored the rest was green, so it had been flushed at some point.
I have decided to go ahead and replace the oil cooler, but I am not sure what to do from there. Should I replace the EGR cooler or wait to see if it is needed?
I am not getting any oil in the water, not getting any water in the oil, not blowing white smoke from the exhaust. I pulled the EGR valve and checked that. The soot in the EGR valve is not mushy wet, but does have a moist feel to it. It feels like very gritty grease. The best I can describe is it is damp.