Since your in Ok and we got nice weather for the weekend I would suggest you buy a couple bottles of the flush (I would get 3, but I'm a little OCD about this sort of thing) Drain the cooling system and flush it all out with water so you get all the basic nastiness out that's sitting in the coolant. Pour the flush in, top it off with water and go for a drive. Let it really run through the motor. After driving for a while go home, drain all this, flush with water to get out any residing contaminants and then add bottle #2 of flush and top with water (maybe a gallon of coolant if it is going to be hot over the weekend and you plan to do alot of city driving) and leave it for the weekend. Drive the car as much as possible over the weekend so it has plenty of run time. Sunday drain it and if you got a 3rd bottle or if it isn't completely clean repeat the process and by this point if the insides aren't clean you need to just get a new engine.
j/k j/k By the end of the 2nd or 3rd flushing job you should be able to fill with coolant, make sure you keep it 50/50 mix, and not have anything to worry about. I would bet the previous owner drained it and tried to flush it with water and then just added green coolant. May have not even thought about it and left the old dex in the overflow jug and the sludging just happened in the jug. Never know. Previous owners do all sorts of silly things with their cars lol.

Good Luck, let us know how it works out.