I have a 2017 Sienna, with the then new, 10 speed transmission. When on a hill I often downshift to 3rd gear to make use of dynamic (compression) braking and save the brakes a bit. Is this OK or is it a No-No. Is there a risk of damaging the transmission. If it isn't OK, why are the options of down shifting even there. Thanks for your expertise.
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On long, steep, mountainous descents, it can prevent your brakes from getting red hot. Otherwise, it's not really worth it. It won't break your transmission, but it does cause a bit more wear.
It is ok to do and will not damage the transmission. I do it myself.
Since you have the 2017 Sienna with the (unproven) 8-speed AT - jury is out on its long-term reliability/durability - I would recommend minimizing (or not even performing) engine braking as much as possible to mitigate potential wear and tear to the transmission.
You put strain on transmission and in long run, it will damage the transmission. Remember that brake is cheap but the transmission is expensive.
By the way, your Sienna cannot "compression brake". That is something usually only big rigs and some diesel trucks do.
I actually opened the Toyotas owners manual and it refers to S4, S3 & S2 for engine braking. The transmission is an HF80 and was new to the 2017 Sienna. A local Japanese auto repair shop said. Hey, if you have it in cruise control it will hold that speed, even down a hill. That hadn't occurred to me to try. But I will. Most of my driving is on local Seattle hills
yes, engine braking is the correct term. Compression brake or "jake brake" works differently.