[wingnut60]

If the oil gets 'milky' looking--kind of like a chocolate malt, you have a problem with the oil cooler leaking into the cooling system. It is a serious problem and costly to fix.
If you are losing coolant and can't find a leak, then you might have a head-gasket leak, or the water pump housing has cavitation pinholes in it.
Both of these are not easily self-diagnosed, needs to be looked at by a competent mechanic.

First, hope you are replacing the coolant with proper coolant, not water!!
There are TSBs from Ford on early '08 6.4L engines about the radiator, hoses, and a 'venturi tee' mod that helps keep the water pump housing from developing pinholes. Have you ever checked the nitrite level on the coolant? Very important component of these cooling systems.

If you were me, with these symptoms, I would seriously consider upgrading to a 2010 450 if you like the vehicle. Or to a 2012-13-14 450. You are probably looking at a minimum of $1500 repairs, and maybe up to $3-4 grand--even more if head gaskets are leaking.
Joe