2003 impala has non oily rust colored sludge that builds up at radiator cap

SOURCE: Overheating

it could be air lock , very common on those engines, more likley is a slight headgasket leak

SOURCE: Re: Dex-Cool in 1999 Lumina

The major reason for using dex-cool is that antifreeze is often neglected as a service item. If neglected, it becomes acidic over time which is particularly important with the use of aluminium in most major components which are served by antifreeze. Acid causes etching and premature failure of gaskets and permanent damage to blocks, heads and especially the radiator and heater core, which are not vert thick to begin with.
Since "normal" antifreeze acidifies faster than does dex-cool, mixing both together causes a change in breakdown rate...the more "normal" coolant added, the faster it breaks down.
Both coolants do not react chemically with each other and therefore retain the same ability to prevent boil over and freezing.
In reality, normal coolant can be used with little or no effect on components, provided that you change it every year. (that's where the EPA gets involved) They do not want people dumping this stuff into the ground, so they push for extended service intervals. Since dex cool has a longer service life, that's what they want you to use.
As far as how you chasnge the coolant, it is always a good idea to flush system during a change, regardless of antifreeze type. This removes solid particles which can plug radiator core. Radiators are much smaller than their older counterparts. Loss of even a small portion of cooling area can cause overheating. Engine running temps are also higher than older vehicles (heat=power output). Therefore most systems have a much smaller margin of error than before. What I find though is that regular flushing can also be bad for the system as the chems eat away at metal components in order to clean. I recommend regualr or dex cool antifreeze systems be flushed with water only, once a year, but retaining what you removed and filtering it through a coffee filter, using it during the year to "top off" the system. To retain effectiveness of coolant, use a hydrometer to check concentration, adding new coolant to bring reading to proper protection level.Then, every three years, do a complete chem flush and change entire batch. This keeps your system clean and the epa happy!

SOURCE: Radiator hose leak and anit-freeze types

I've worked with GM for 30+ years and I have to say dex-cool is one of the sorriest coolants I've ever seen. It lasts no where near as long as the old green or universal coolants and it also causes the coolant system to corrode. I replace all coolants at my shop with the global coolant unless the vehicle is under warranty for that purpose.
To answer your question, the global or universal coolant is a far superior coolant to dex-cool regardless what GM says and is kinder to the coolant system.