Treat this as a blown head gasket. Coolant is being lost with no external leak, and milky residue under the oil filler cap supports internal coolant mixing. Assess how much engine damage has occurred and decide whether the truck is worth repairing before proceeding.
The owner of a 1987 Chevy 1/2-ton pickup with a 305 V8 reported milky-looking oil under the filler cap but not on the dipstick, and later added that the radiator and overflow bottle empty within about 100 miles with no coolant visible on the ground or outside the engine. The expert identified the symptoms as a classic blown head gasket and said the extent of damage and whether the truck is worth fixing are uncertain.
