I have a 2000 Sienna with 113k miles. I have not had any engine sludge problems (yet). Since it was new I've been using full synthetic oil, (first Castrol Syntec, and now Mobil 1) keeping to a 5,000 mile change interval as best as I could. The engine burns zero oil. I do notice the oil is pretty dark when I change it, but no evidence of gelled oil exists. It seems to me Toyota really should have required synthetic be used in this engine, but probably was afraid of how a significantly more costly oil change would have been received by critics and the public. If what I read is true that Toyota raised the operating temp. as high as 260 degrees F, then it was irresponsible to not require a synlube.