While trying to decipher if it matters if I use 5W30 over 15w40 for my next engine oil change I got this in http://www.carbibles.com/engineoil_bible.html.

Toyota sludge problems.
For their part, Toyota have the dubious honour of having the most complaints about sludge buildup in their engines - 3,400 at the last count. At the time of writing there is a class action suit going on against them. Details can be found at www.oilgelsettlement.com

Toyota seems to have lost the case. Since the cars cover older models (pre-2007) then it would be of interest only to owners of older Toyotas and Lexus.

Kung 2007 or mas luma toyota mo it would be a good idea to be diligent in changing your engine oil. Unless interested kang makita ang "black death" or sludge.