From what I have read, there are some reports here where the owners have properly maintained their engines and still got sludge. It doesn't appear to be maintenance related at all, which leaves some sort of mechanical failure as the cause or some operating condition that wears out the oil extremely fast.
I am the owner of a sludged Sienna that WAS well maintained. Toyota came out with a policy to help owners of Siennas, Avalons, Camrys, Solaras, Highlanders, Celicas and some other models, because there is a serious problem of low mileage sludge in these vehicles, beginning in 1997, and until 2002. In April 2002, Toyota changed the design on the engine to help prevent sludge. The design change involved the PCV system and the engine head.
Some engine failures due to sludge in Toyotas could be due to lack of maintenance. The percentage of poorly maintained Toyotas getting sludge should mirror the percentage seen in the industry. But Toyotas seems to have above average problems with sludge. Do a web search (google.com is a good search engine for this) for key words "Toyota Engine Sludge". Compare what you get to when you do a search for "Honda Engine Sludge".
There does indeed seem to be a mechanical problem in these vehicles. Toyota is not fixing the PCV or engine head until AFTER you get sludge, so remember to take extraordinary care of your Toyota and watch for signs of sludge - MIL illuminating, smoke in exhaust, signs of sludge on dipstick, heavy consumption of oil at very low miles.
Toyota is telling owners to be sure to check the oil frequently (at least every time you stop for gas) so you can watch for these signs.
Yes, you're absolutely right, Toyota did mail a letter out to all owners of the vehicles you listed with the involved engines. Perhaps Toyota, being the smart company that they are, would like to make the public aware of a potential problem instead of keeping some corporate secret that everybody at the company is "in on," yet none of the public are privy to. Toyota is the number one money making auto manufacturer, and I'm sure they would like to stay that way. Even if that means buying an engine that they should not have to now and then. By taking care of customers that have either not taken care of their vehicles or been extremely unfortunate, they obviously look more dedicated to their product than if they were to completely deny that the circumstance could not exist at all. My 1997 Camry with 180,000 miles on it runs extremely well and has not even come close to exhibiting the symptoms people have been describing; nor has my aunt's Sienna, my grandmother's Camry, or my friend's Highlander, among the dozens of people with "involved" vehicles. And as long as they continue to maintain them, they probably never will.
There is significant evidence by the way of reports on the Internet that Toyota did not make this information about its sludge policy available to the owners of the subject models.
Many people, some original owners at their original address, are only aware of the problem AFTER they have experienced it themselves and AFTER doing a search on the internet. Try doing a search for Toyota Engine Sludge using http://www.google.com for more information.
Other good resources are listed below.
http://www.yotarepair.com (even has a "Sludge Zone")
http://autos.msn.com (see Ratings and Reviews for models covered under sludge policy)
http://www.autosafety.org (this site has info on the sludge policy, a copy of the letter that owners SHOULD have received, and also has compiled complaints of owners)
http://www.edmunds.com (go to Town Hall and do search for "Sludge" to find reports about engine sludge)
http://www.nhtsa.gov (see reports filed by owners that have been made to this government agency in charge of mandating recalls)
I know of instances where car owners (Chevy Caprice and Dodge Van) did not change their oil in over 100,000 miles, and the engine never developed sludge. Both of those vehicles went on to go over 200,000 with the same treatment.
I know it sounds unbelievable, and I even have a difficult time believing it myself, but I witnessed both these instances in horror myself. For some reason I cannot comprehend, the owners simply did not believe in changing the oil. This is not the way to treat a vehicle, but these engines somehow managed to last.
I myself too hastily bought a car ('93 Ford Festiva) without giving it a good enough looking over that had 25,000 miles on it, and when I changed the oil, discovered the engine had the original oil filter and presumably the original oil. The oil looked hideous when I changed it and hydraulic valve lash adjuster failed at 30,000 miles (I'm sure as a result of the oil not being changed), but there was never any sludge.
My point is that it is not normal for an engine to develop sludge in the short amount of time Toyota engines do, even if the oil has not been changed.
I have a 2000 Toyota Sienna with 115,000 miles on it and recently had to have the transmission rebuilt. Does anyone else own this model and had transmission problems with it. I purchased a Toyota because of their reputation in being a dependable vehicle for at least 200,000 miles or more. We regularly maintained this vehicle and feel that transmission breakdown should not have been an issue even at 115,000 miles with regular maintenance. My husband drives a Honda accord with 188,000 miles on it and has never had to replace the transmission. What's up with that? Makes me think Honda is the way to go next time we're in the market for a new vehicle. Any remarks would be helpful.