I'm a little miffed on how the obvious is missed. This toyota issue on sludge is and has been an on going problem. No, not all sludge issues are because of neglect. No, not all sludge issues are because of coolant ingression.
Toyota is the ONLY engine that has those gears on the heads. As Gman pointed out, those gears are notorious for shearing down a normal oil in viscosity. We have evidence that in the normal v6 sludge engines, a 30wt oil(castrol gtx) will shear down from a 30wt to a 20wt just inside of 3500 miles confirmed with oil analysis and no evidence of any contaminants. There is also evidence that shows anyone that did 3k drains did not in-fact have any sludge issues. There is also evidence that only certain engines had this problem and most times it was ones that had tight engine compartments, where air flow was lacking and had smaller oil sump capacities.
Toyota was trying to stay up with other dealers when it came to extending oil drains but failed to recognize just how their engines would affect the oil.
Yes, in my research, I have seen a brand new 12,000 mile engine, totally locked up and blown with sludge at the dealership.
There are certain steps to keeping a toyota from sludging. If standard oil is used, do not go beyond the 3k drain intervals. If going to 5k on up, then a good oil such as schaeffers, amsoil, m1 or redline should be used and those oils will not shear out like most conventional oils.
To those with toyota's and no sludge, I'd put some on the fact that you either following the 3k drain or are using a better than normal quality oil. The evidence has shown that all these sludge engines used weak otc oils and that if anyone had a sludge issue with say amsoil, it was due to trying to do 20k drain intervals.(there was one case like that).
Anyway, That's the skinny on the basic sludge issue. Nothing hard to understand.