Diesel Bombers - Thick Oil in Water

I have a 92' Ford F-350 IDI. I have had the oil cooler replaced twice now, but the water isn't showing up on the dipstick or even out the tailpipe. When I open the radiator cap after vigorously running the truck, there is very thick oil residue in my coolant. Is this a bad head? Or perhaps, it's something even worse. I've flushed the system numerous times using everything from Dawn to Prestone Radiator Flush to stop this problem. None the less, the thick oil residue still returns. While dealing with this problem, I've put on 6,000 miles and 5 oil changes, so I know it's not from lack of maintenance. I just want my problem truck fixed. I do not want to depart with this beautiful 7.3 motor. I've worked for a carnival for 30 years, this is one of few trucks that can run on vegetable oil and diesel mix (bio-fuel.) You just change the fuel filters a little more often. By the way, the bio fuel mix is about 75% oil and 25% diesel. My trucks worked fine until they were totalled or what have you, this is the only one that's given me this problem. The truck has a RMP motor. The injectors have all been replaced within 2 months. Please help.