He says it's "like thin Vaseline", which means it is some sort of emulsion. Honestly, I was expecting a blockage which was more crystalline in nature.
Mine was purchased new, and has the same problem. So it is likely not a problem with an incorrect antifreeze added by a previous owner. One thing I have wondered: does the replacement OEM heater core have a different part number, or did Mitsubishi change the antifreeze they are using in order to solve this problem? The only part number I can find is 7801A953, and I can't find any mention of an earlier part number that this part number superseded. Did they just change how these heater cores are manufactured without changing the part number?
Sheeesh...what is it with new cars and 'sludge problems' nowadays??