What is themost important maint. with 140k miles

Are you asking why do you need a thermostat at 140k miles? You don't, unless you are having trouble with it. Overheating, sometimes not warming up? If your thermostat is faulty, that is what will happen, you car will overheat or not warm up, cabin heater may not work, but if the temperature gauge (I know that '02 Avalon has a gauge not an idiot light) if it goes to the middle and stays there, even if you have 3 Football players in the back seat, your 300 pound mother in law in the front seat, and a trunk full of bricks driving up Pike's peak in summer and the gauge stays in the middle, or even a little bit higher than middle, you have no worries about thermostat

Thanks for your reply. I'm not having any problems at all. I want to get 200k out of the car and want to know how to get there maintenance wise in addition to changing the oil.

You will get 200,000 from that car with, as you say, proper maintenance. Get your VIN# ready and go to: http://www.toyota.com/owners/web/pages/login that is the Toyota's owners website, register there, it will show you lots of things, specific to your car, and I mean YOUR car not the model... prior service records, what you need to keep up properly, everything. Don't worry about the thermostat. There are other more important things. Use a good synthetic oil, yeah it's expensive, like Mobil1 is $10 to $12 a quart but WORTH IT and a GOOD QUALITY filter, and you do not have to change it every 3,000 miles. 5,000 is even more often than necessary, especially with that oil. That is about the most important thing you can do. I highly recommend you register there.