Recently I replaced the LIM and UIM gaskets on a 3.8L. The 3.8L was in a recently purchased Bonneville I purchased at night in the poring rain at a Virginia new car dealer.

When I brought the Bonneville home, I noticed coolant pooling on the ground. Later took out the pressure tester and found a leak on the passenger side of the engine. Could not clearly tell if it was a coolant elbow or LIM gasket- I replaced them all.

I also replaced the coolant overflow container. I cleaned the coolant overflow container on my Son'* S10 last year- it took me all day. I asked myself why are the insides of this newly purchased Bonneville and the S10 so nasty- yet on my 2001 SSEi with 230k miles it is spotless inside?

I am speculating that when coolant leaks and/or air gets into a cooling system running DEX, over time a lot of souring of the coolant takes places which in turns leaves nasty residues and deposits. On both the new Bonneville and S10- they were purchased with leaking LIM gaskets and likely were leaking for a long. On the SSEi, although the highest mileage of the three, the LIM was changed very early and there was never a coolant leak or air introduced into the cooling system.

Here are pictures of the before and after coolant overflow tank. I sense a dirty coolant tank on a DEX coolant vehicle might be a indicator of a vehicle being run with a coolant leak over many months/ years.

Also I am inserting a link to a Bonneville for sale on CL, I sense it to has had a long term coolant leak looking at its coolant overflow tank- all possible signs of neglected service and maintenance. The seller of this Bonneville cleaned up the engine cover and air box- but the coolant overflow tank tells a very different story of how this car was (not) taken care of:
https://philadelphia.craigslist.org/cto/5445526793.html

Have seen reservoirs from all engines, with this residue......called TAC about it.......they said, sometimes it is caused by adding too much coolant sealing tablets at the factory, or after repairs(they recommend tablets when the system is opened).....I'm sure your theory, is a possibility also....