A lot of folks think "antileak" antifreeze
must somehow be better. Which is how it was sold for years. But most of the antileak stuff does that by adding sodium silicate, aka "waterglass", which forms a glassy crystaline coating on all the interior parts
. And prevents full heat transfer, and clogs small passages, but that's neither here nor there.
It COULD be that someone used that stuff once upon a time, and some of the waterglass is breaking free and coming up as crud. There must be some simple ways to test the crud, see if/how it burns, smells, tastes, what dissolves it. If you really want to chase it down, give a sample and a case of beer
to any college chemistry student and ask them to get it in the lab. <G>
Or as suggested, do a thorough flush, try to clean it all out, start fresh and see if that's all it needs. FWIW only distilled water and a/f should be put in the cooling
system. Sometimes there are enough chemicals in tap water so that it can create problems in a cooling
system, and (at least here in the lower 48) distilled should still be something like $2/gallon in the supermarket. Or de-ionized, used in steam irons for the same purpose, no chemical buildups.