I’d have to agree with the rest of the guys on this issue, don’t shift it manually unless you need to (like going down a steep hill, which was already mentioned). If the car has a gated shifter, I probably still wouldn’t downshift unless I needed to. I can tell you that some folks with automatic trannies have had a manual valve body installed, so the only way you can shift it is manually; you just don’t have a clutch pedal. Of course, this was usually done by guys who had high-performance cars with souped-up engines, wanting to “bulletproof” their transmissions.