There is a little pot or cylinder in the center of the thermostat. that needs to be pointed down into the coolant.
Your motor has a crankcase vent system that uses a pipe terminated underneath to create a partial vacuum going down the road. Fresh air is supposed to enter the oil filler cap and be drawn out down below. When the truck is stationary, non of this happens and the fumes come out the top. All of them produce some blow-by past the rings and this is what you see. The more worn the motor is the more fumes it will produce.
