1998 Ford F-150 milky oil cap

When water mixes with oil, that happens, but if it is only on the cap, I don't think you have a head gasket problem unless you are adding coolant on a regular basis or there is white smoke coming from the exhaust. To me, it sounds like the PCV valve may be plugged. The PCV (positive crankcase ventilation) allows crank case pressure to be recirculated back through the intake. If it is plugged, then condensation can build and do that to the cap. I would try replacing it to see if it helps. It is a cheap part.

Joe