If you have the pink Dexcool antifreeze in it then that is the reason. You can go on the web and there are several class action lawsuits against General Motors concerning the issue.
Yes, stop leak turns into a thick substance to plug any leaks, when it ends up in the overflow bottle it turns into sludge there.
Coolant is old.
red
Mineral sludge from tap water. A good radiator flushing will help and then use proper coolant to help keep the sludge building up and blocking the radiator.
Time to replace the head gasket.
You can use plain water to get sludge out of your car radiator. You can use Marvel Mystery oil to remove sludge from the oil pan.
the so called sludge is most likely some sort of cooling system sealant.
Time to replace the head gasket.
if sludge is keeping oil from draining away from valve guide a cylinder can suck in oil and cause plug to misfire
Car radiator flush sold as most automotive shops
The most likely cause of this is a blown head gasket dumping oil back into the coolant passages. The oil will mix with the glycol in the antifreeze and make a thick yellow paste. It is very important to flush the cooling system after the repair or this sludge will cause heating problems.
Get one of those flush kits from an auto parts store.