Q: When should I change timing belt and transmission fluid? 2008 Toyota Sienna

Hello - your Toyota does not have a timing belt that requires replacement periodically. It has a timing chain, lubricated by engine oil, that lasts for much longer. Changing your engine oil and filter regularly is your best maintenance for prolonging timing chain component life. As for automatic transmission fluid - although many vehicles with automatics now come with "lifetime" fluids, my personal belief and practice is to change it every 75-100K miles, with factory fluid, if you intend to keep the vehicle for more than 100K miles. It is an inexpensive investment. I recommend a transmission fluid and filter replacement service by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, and perform this maintenance for you.