[bill dowd]

Both of you have valid ideas Toyota made changes the engines in 2002 to help keep down sludge due to the engine design for the smog system. Another reason sludge builds up in the motor is some people just drive a couple miles a day/week and don't let the engine get hot enough to get out the water/vapor inside the block and this will also build sludge. As far a changing synthetic oil I go 12-15,000 miles on my LS400 lexus and soon as I get out of warranty I will go the same on my 2006 camry. BMW Mercedes Corvett and all the high priced cars costing around 75,000 and more all come with Mobil one or some other synthetic oil and go 15,000 miles between oil changes. It takes about 7-8 miles to get all the vapor out of your block everyday and also to the engine oil to go thru the oil filter once . This information come from different car manufactures I also use mobil one extended which is supposed to go 15,000 miles between oil changes and I also check my oil level every 5,000 miles some cars will consume oil. I also read that dino oil has made improvements to be able to now go 7500 miles this is why Toyota recommends 5,000 miles changes. I put 429,000 miles on a Toyota Cressida using mobil one in everything motor,trans,rear end and power steering and nothing ever wore out in that time.