It has to be the new low-viscosity type of ATF that only a few people besides Toyota sell; Amsoil is one, haven't heard about Valvoline making one but they could well do.
If you only drain and fill, you are only changing a fraction of the fluid, most of the old dirty fluid will still be in the torque converter and the rest of the hydraulic system. If you want to change it all then will need to flush the whole system.
i asked the dealer about ws and he said max life is what they only use. as far as drain and fill goes. i have two toyota, one is a tacoma w/ 267k on it. i will only drain and fill that one. the fjc i may flush. i don't post on here, but i do read alot and there seems too be a big brew ha about drain and fill vs flushing!