1998 Ford F150 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

i had that same issue on my 1990 F150. Check dipstick and confirm your oil pan is not filling with coolant and if so you have trouble. If it is just in oil cap test for cylinder head gasket failure. Mine failed passenger side 2nd and 3rd cylinders, head gasket was allowing exhaust and oil into coolant but not on a large. If you are competent you can change top end gaskets but you will need a hoist as the heads are heavy. It is easier to lift them out but putting them back on took some doing as they would slide off. Use new cylinder head bolts as old ones have stretched and weakened. There was many annoying things in this job. I should have paid someone with a shop but money is always short. The air plenum too cracked a corner at bolt hole.... found one at junk yard..