To find the service records for all service that has been performed on your vehicle at any Toyota dealership, register your car for FREE at
www.toyota.com
You will need your Vehicle Identification Number (VIN).
You will also have access to a online version of the owner's manual and any recalls for your vehicle.
A "drain & fill" gets, at most, 1/2 of the total fluid present in the transaxle system.
A fluid exchange (often called "flush" but is just an exchange with new fluid) gets close to 90% of the fluid.
A fluid exchange can be done at most "quick lube" places.
Most will use the fluid that you bring....but make SURE that they actually put YOUR fluid in.
The Dealership likely will do the fluid exchange, if you specify that this is what you want done.
I use Redlin D4 in my Toyota, which calls for a Dexron III fluid.
The D4 is also recommended for the T-IV application.
Mobil 1 is also well respected and easier to find.
I get my Redline online.
Amsoil also has a good reputation.
I have not read about Royal Purple in the T-IV application....however, if they list it in the recommended application list on the bottle, then I would trust it.
If you are set on a drain & fill, then I would do at least 2 of them......drain & fill, run the engine......and then repeat.
Not as complete as a fluid exchange, but you will get enough of a fluid change that it will be close.
As mentioned above, adding a Magnefine filter to the cooler line going from the cooler INTO the transmission would be a good addition.
They catch a LOT of stuff that the in-pan filter does not, and they have a big magnet that all the fluid flows over, catching a lot of the metal (clutch) dust from the fluid.
The end result is that your fluid stays cleaner for longer, which is great for keeping the fluid valving clean.
The magnefine filters to a nominal particle size of 30-35 microns, which is 2 to 3 times smaller than the BEST of the in-pan filters.
My older Toyota (2003 Sienna) only has a nylon screen inside the in-pan filter, so the Magnefine was a MAJOR step up for me.
Changing the fluid, as you are thinking, is a major way to reduce the particles in the fluid.
Most of these particles are normal clutch wear material.
The problem with not changing the fluid is that these particles can clog the fluid valves and also cause wear to the fluid valve and valve seats.
Fortunately, the transmissions in Toyotas have a reputation for being quite reliable.