If raw air were the culprit, all Dex-cool systems in every vehicle with a recovery or expansion container would suffer from sludge.
They don't.
And... reverse flow V-8's like the mid-1990's LT1 & L99 would be immune to Dexcool sludge.
They aren't.
I'm not positive that a blown head gasket will cause sludge. That's the rumor but I've had a Castech cylinder head failure and a blown head gasket on different vehicles. Neither had sludge when I yanked the heads and both had Dexcool in the cooling systems.
I'm going to say what your dealer didn't. Dexcool turns to sludge in the presence of contaminants. Likely someone poured the wrong stuff into your cooling system.
The big three offenders in order of likelihood. 1&2 are a toss up.
- Someone poured in some old "green" Silicate coolant.
- Someone used one of many "compatible" and "universal" coolants that aren't quite compatible and definitely not universal.
- Someone used coolant additives (pick your choice of bottled cooling system snake oil).
I've read rumors that tap water might have enough contamination to make Dexcool sludge but I seriously doubt the veracity of those claims. While it ain't a good idear to use tap water in your cooling system it's not the end of the world either.
My feeling on the matter is that you use what the machine came with and you won't likely have problems. Prestone Dexcool 50:50 is cheap and you can get it at WalMart. I keep a jug of it around to top off my GM Dexcool rigs.
I have some Prestone Green silicate stuff for the Scout and various other machinery from the 60's & 70's.
Where you get into trouble is when someone else works on your rig or a family member uses it and helps out by topping up the coolant with the wrong juice.