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The oil in the coolant is caused by a bad oil cooler and the fix is to replace the oil cooler.
The good news is Ford has a recall on the cooler and you should be able to have the repair done at no cost
OWNER NOTIFICATION PROGRAM
Certain 1999 & 2000 Model Year F-250 Super Duty Through F-550 Super Duty, 1999 F-Super Duty Stripped Chassis & 1999 E-150 Through E-450 Super Duty Vehicles with 5.4L or 6.8L Engines-Engine Oil Cooler Inspection/Replacement
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Here is a copy of the letter Ford sent out.