The regular green anti-freeze, its anti-corrosion additives only last 2 years. Once those additives are used up, metal parts in the cooling system can start to corrode. Rust is brownish and can look muddy in solution with water.
As well, the silicates used to protect the aluminum in "Green" anti-freeze, is a derivative of sand. I'm NOT sure, but I think the silicate additives can degrade into a sand like residue.
Either way, you need new coolant every 2 years, regardless of mileage.
I'm currently degunking my cooling system, running distilled water with Prestone Flush (the citric acid that you run for 6 hours), it seems to work and really cleans out the gunk, even the coated on gunk you can't ever reach.
It won't do miracles, although my old radiator looks much cleaner, and the scale has started to break up a bit, there is still a lot of scaling in the radiator. I'm replacing the radiator, I'd rather use the flush on the old radiator, once I'm done I'll swap in the new radiator.
Unfortunately, many of the manufacturers have switched to a new "Universal Anti-Freeze" that works in all models with all colors of anti-freeze. From what I've gleaned from a little research, this new "Universal Anti-Freeze" is just GM Dex-Cool dyed another color. In fact, in the fine print, it says you have to completely drain and flush your cooling system before using it. So be careful when picking out anti-freeze, anything claiming to be "Universal" or Long/Extended Life is likely some sort of derivative Dex-Cool or G-05 (OAT and HOAT technology). Your cooling system was NOT designed for those fluids, its just the opinion of the manufacturers that if you flush your system well, their fluid will work well.
Look at the ingredients of the anti-freeze bottle, old "Green" anti-freeze uses Silicates and Phosphates for corrosion protection. If you don't see some form of those in the ingredients, then likely its some new derivative of Dex-cool or G-05.
Dex-Cool and G-05 have NO corrosion protection additives, instead they use additives that promote the metals to form their own natural layer of corrosion as protection, and preserve that natural layer. Thus their additives last much longer. I "THINK" the ingredient 2-hexane, something like that, in Dex-Cool is what caused all the nightmares you heard of about Dex-Cool (OAT), G-05 (HOAT) doesn't have that ingredient and even use a little bit of the silicates/phosphates for protection.
I've seen several opinions, that if your going to switch, that G-05 is better to go with than Dex-Cool for cars designed for the old "Green" anti-freeze, it should work better and be more compatible.
Personally, I'm using the original recommended fluid, It appears the NAPA generic and ACE hardware Generics Anti-Freeze has Silicates and Phosphates in them. Zerex makes an original "Green" formula, but I can't find it for sale anywhere in my area.