I wanted to add my experience with my 1998 Sienna, purchased new and now with 158,000 miles.
I first noticed I had the sludge problem at about 50,000 miles when I noticed gelled oil (consistency of jello) just inside the area where new oil is added. There were no other indications, and I never saw any performance issues or "blue smoke", etc.
I had made regular oil changes with Pennzoil regular oil, but when I took it to the dealer, they and Toyota insisted there was something I was doing wrong. In any event, I paid $2000 for them to pull off the valve covers and the oil pan, which revealed no damage to the engine, and they also performed a flush of the engine. During the flush operation, they must have spilled something on my inside back floormats, because there were areas that looked like "trails" of liquid where the fiber appear to be chemically melted -- but the dealer denied this and refuse to take responsibility, even though I knew the mats were perfect when I took the vehicle in -- yet another bad experience to add to all the other bad experiences I have had with dealer arrogance and irresponsibility (but that is another story).
About a year after the above work, I received notice of coverage for this type of event, and sent in documentation of my oil changes, I received a check for $2000 from Toyota. I was told to continue using non-synthetic oil, and to be sure to change the oil every 3000 miles.
At about 75,000 miles my oil pressure light came on, flickering at first, and then on permanently, and I took it back to the dealer. They pulled the valve covers and oil pan again. The area under the valve cover looked perfect with no problems. However, the oil pan contained many little pieces of what appeared to be carbon deposits, and the oil pickup tube screen was covered with these same deposits. They appear as tiny little hard black pieces that will break if you press on them with your fingernail, and the size of a typical piece is a little smaller than diameter of a pencil lead, and there are hundreds of individual pieces.
The dealer cleaned out the oil pan, and flushed the pickup tube, and all was good. However, since then, like clockwork, every 30,000 miles or so (about 2 years or a bit longer) the problem returns. The second time, the oil pressure light came back on. Each time I did the work myself, pulling the oil pan and cleaning out the deposits from the pan bottom and the oil pickup tube screen. After the second time, I just began doing it every 2 years or so, and everything has been fine. The bottom line is that there is no permanent solution that Toyota recommends for this problem, it is simply inherent with this engine in some instances (obviously not all).
So mainly I just wanted to add my experience, and state that in many cases with the oil pressure light active, I suspect the problem is exactly what I have, and that all the other things a dealer may want to do will be fruitless and needlessly costly. Pulling the oil pan is a relatively simple and cheap operation (it takes me about 2 hours start to finish, and I don't have a lift) and it is simple to see whether the pickup tube screen is covered with deposits.
At 158,000 miles, my Sienna still has good acceleration, does not burn oil, and gets good MPG.
By the way, I live in Texas, with hot summers and mild winters, so any theory that says that the problem relates to cold weather is incorrect IMHO. Last, but not least, anyone who thinks this problem is due to not changing the oil frequently enough is just plain wrong -- I don't know what to say to them other than just search the web for people's experiences, many of them very careful to change their oil every 3,000 miles as I do, and the problem still occurs, and further Toyota has acknowledged the problem -- what more can you possibly want to accept something as the truth regardless of your personal experience?
I hope this helps anyone having this problem, since I assume that many affected Toyota vehicles will continue to live on for some years to come, many with new owners when the vehicles are sold.