The best way to see if you have a sludge problem is to take off the front valve cover. You'll see a gel like thick oil substance and if you rub it between your fingers it will be gritty. My sisters mechanic mistook it for metal fragments and told her her engine was about to blow. Diagnosis wrong, effect correct. Most mechanics won't recognize it for what it is. If you have a lot, you can have the engine cleaned out but it's expensive. For prevention change oil often (3000 mi)and use synthetic oil. Toyota will repair engines with sludge problems that are in a certain build date range for up to 8 years for the original owner. The cleaning they did on my sisters Sierra at 32000 miles turned out to be a very temporary bandaid. I haven't gotten final word on whether or not they will honor the engine replacement for her van yet. If not, my view of them is likely to sour just a bit. Then again, Toyota is known for making great cars, not promising great service. The later opinion may change as well, providing they replace the engine.