Match, confirm that your '08 sienna uses Toyota WS ATF. It is a long life fluid, but does need changing periodically. The term "flush" machine is used for a couple of different type machines. One is simply a fluid exchange unit with two tanks and a diaphragm between them. New fluid is poured into one tank and the machine is connected to the transmission cooler lines. When the engine is run old fluid is pumped by the transmission's internal pump through the cooler inlet line, into the machine's empty tank, presses against the diaphragm, and pushes new fluid through the cooler return line back to the transmission.
You can do much the same thing yourself. Drain the transmission pan. Renew the filter if you want. Refill the transmission back to normal with new fluid. Disconnect both cooler lines ('cuz you probably don't know which is the inlet), aim them into a bucket, and run the engine briefly until one or two quarts is pumped out. Stop. Refill the transmission with new fluid. Run another one to two quarts out. Stop. Refill. Repeat until the fluid comes out bright red. Correct the level. You're done.
You can do much the same thing yourself. Drain the transmission pan. Renew the filter if you want. Refill the transmission back to normal with new fluid. Disconnect both cooler lines ('cuz you probably don't know which is the inlet), aim them into a bucket, and run the engine briefly until one or two quarts is pumped out. Stop. Refill the transmission with new fluid. Run another one to two quarts out. Stop. Refill. Repeat until the fluid comes out bright red. Correct the level. You're done.
