As a few owners have found out the hard way, on some engines you can get a coolant leak into the engine (which tends to produce white exhaust) or you can get an oil leak into the cooling system.
GM had done the recall on mine where they replace some nuts and add sealant to the cooling system. Fine.
A month later, I saw this brown residue in the car's coolant reservoir. Oh-oh. If that is engine oil leaking into the coolant, then I am in trouble. At first, I guessed that it was from the sealant that had been dumped in. But then, I wasn't too sure, so I decided for a little laboratory analysis, on advice from my brother, the analytical chemist.
I extracted some of the brown residue from the reservoir after the engine was hot and all of the fluids were circulating. I had a Prestone coolant tester siphon that sucked it out, and that was squirted onto a piece of expensive filter paper. Well, actually, it was a piece of ordinary paper towel. I got that residue and washed it with water. Then I dried it on the paper. I scraped it off into a clean container that had about one ounce of warmed isopropyl alcohol in it.
At this point, if the alcohol picks up a brown coloration, then that probably means that a petroleum-based substance is present, and that is the bad news. In my case, the brown stuff did not dissolve, and the alcohol stayed clean in color. So, that means mine had the brown sealant only, so I am safe.
Good luck.
---Bob Gross---