That brown stuff is leak/seal tab residue. I went through the same thing on my 03' around 30K or so as far as cleaning it out of the overflow. GM in their infinate wizdom decided this would be good insurance to protect against the rash of gasket problems related to Dexcool and the gasket material used by adding these to the cooling system, at least through the initial warranty period anyway.

 

Do they even make this stuff anymore? I've been looking at auto parts and big box stores in my travels lately and have not seen it.

 

My 2006 Monte Just had it's DexCool replaced at 50K miles..

I had it done because it looked like gravy...

I never had this issue with the green stuff..

 

Dex-cool fails when exposed to oxygen and will then eat at the intake gaskets, especially on 3.1, 3.4 and 3.8 engines. This occurs every 35-40,000 miles.

 

Same here, just put some more in as I didn't realize that I had almost ran out of Dex-Cool. The damn crust crap was built up around the fill line so I didn't realize that it wasn't full until I moved the reservoir to thread my amp cable through the firewall.

 

It would be fine if it was flushed more often. maybe 2-3 year intervals. and i've seen how it just turns to goo when left in a vehicle.. i switched to green in my cars.