I think a lot of people assume is a propylene glycol vs ethylene glycol issue, but I'm not sure that is the case as the two are generally miscible (I have never checked a chart but know in large cooling systems, no one ever really worries about mixing or gelling issues between the two). I'm not super familiar with the various additives in the modern coolants (like the pink stuff), and I've personally never experienced this gelling issue, but my guess would be that when a traditional green antifreeze (ethylene-glycol base, inorganic acid) is added to OAT (organic acid) type antifreezes, the silicates become the problem. Without knowing the chemistry that well, I would suspect that the silicates are either hydrating (possibly through a reaction of silicic acid) or are making a colloid of some sort.