I have a 99 Toyota Sienna whose engine is prone to oil sludge.
There is a class action lawsuit on these engines that span from 1997-2002 and includes the Lexus brand as well - more info can be found here: http://www.oilgelsettlement.com/. I took the car to the dealer and they refused to repair/replace the engine. I am currently fighting the dealer's decision but, in the meantime, I need to maintain my engine as I plan on driving it until it dies. So, I have used a product called Auto-Rx which essentially loosens the sludge which then moves through the engine and into the oil filter. I have no affiliation with this product but just want others to know they can save their engine, avoid costly repairs and keep their car running well, longer. I have been using this product since 2003 when I first learned of the sludge issue. I have my engine oil analyzed through a lab periodically and as soon as sludge starts to develop, I do an Auto-Rx treatment. My understanding is Auto-Rx dissolves the sludge so it can be trapped by the oil filter. More info here:auto-rx.com .