I have a 99 gs300 and was wondering if it is possible to totally flush out the fluid myself or do i have to take it to a dealer that has the machine to do it and what is the total capacity of the transmission???

 

A shop's gotta do the power flush man to get out the torque convertor fluid. Make sure it's the power flush and not the drain and refill. There are some threads out there with specific details to search from. Welcome to CL.

 

There is also a chemical treatment using two fluids to get most, BUT NOT ALL, of the fluid out.