Ford 2004 6.0 diesel engine has oil coming out of water overflow tank for engine radiator

Could it be ATF, due to an internal leak in the transmission cooler (radiator)? Remember to add Supplemental Coolant Additive (SCA) to the coolant and test regularly. Disastrous cavitation problems can develop if SCA is not used. Without knowing more, it's difficult to say what's wrong.

This oil is black coming out of radiator overflow tank so I don't think it has anything to do with that transmission.

Sounds like head gasket leak is allowing engine oil to mix with coolant. I'd take it to Ford for repair.

Why disel cars are cheaper?

Why disel cars are cheaper than normal ones in price?

I would try Egr cooler first. Oil probably black due to soot from exhaust getting forced into cooler.

To get engine oil in coolant, most of the time its an oil cooler problem, oil pressure is higher that coolant pressure which will then force itself in coolant.