Hi just a quick observation/question on my 1989 five litre the radiator overflow bottle has a cold fill mark that no matter if I fill one or two inches above the mark after a few days driving the coolant seems to drop to the cold fill mark .
Theres no coolant in the oil and no leaks , very strange.

 

That's because when the system gets pressurized it expels the excess from the radiator/cooling system. They figured out that x amount of fluid is pushed out when the coolant expands from the heat. X is equal to the amount the overflow tank can hold between the cold fill line and the top of the container. As you are driving, coolant expands, pushes past the radiator cap and fills the overflow tank, any extra you have above the cold fill line gets drained out of the cap, as the coolant cools down it "inhales" the coolant it purged back into the cooling system. Filling past the cold fill line when cold is basically just dumping it on the ground, don't do that.