I have a 2005 Toyota sienna automatic with almost 160,000 miles there was no transmission flush at 100,000 miles I was told that it's counterproductive to do it now so I'm reaching out for your expert opinion.
Never flush an automatic transmission. Debris can easily become lodged in the valve body and your transmission may not shift properly again.
Most of this is mentioned in our FAQ, hyperlinked here: https://carkiller.com/scottykilmer/howto/atf/
Automatic transmissions are powered by hydraulic action and friction. There's friction material in the transmission that allows it to shift. The usual standard is 5 years, or 60k miles. It's the same as the warranty period when you first bought the car. If you've neglected the transmission for this long (160k miles is almost 4x longer than recommended) , leave it alone.
there was no transmission flush at 100,000 miles
100k is too late. Every 50k is when you should replace the fluid. Drain & fill, never "flush".
