Change your gearbox oil
I was on the fence about doing this for awhile. My 2020 Grand Touring, which I bought about a year ago, has 164k miles and I don't know much about the gearbox service history, just that the car was regularly serviced at 5k intervals through around 130k miles. I bit the bullet and did the fluid change last week. The fluid was black instead of blue and smelled burnt. (Check the photo)
"Lifetime" fluid may not work out for you if you're planning to keep your car past 150k miles. I'm planning to drive this car as long as possible.
If you're a DIY-er, you can do it all for about $100 USD (filter, fluid and ATV). Make sure to get FZ fluid. Best price I found was AISIN FZ fluid on RockAuto. I got the WIX filter from there too.
My car definitely had the gearbox oil changed at one point since the RTV form-a-gasket around the pan was silver (not orange) and there was some evidence of orange RTV on the bolts. So at 164k this was the second fluid change. I was glad to see this, I can't imagine what the fluid and filter would have been like if this was the first time. I was lucky to see very little metal on the magnet in the pan.
I did have a few issues that YouTube videos did not address:
There's even less space to work around the transmission dipstick/fill hole if you have a turbo car. You'll want a funnel with a long flexible hose to avoid battling other hoses in your way.
Not sure if this is a Mazda6 face-lift thing or GT thing but the gearbox had about 5.5qts of fluid in it (instead of the 4-4.5 like YouTubers were finding). At. 5 quarts it barely registered on the dipstick. I ended up buying another quart to make sure the fluid level on the dipstick was correct. Checked it at temperature and its right at the top of the acceptable range.
Good luck to anyone else who does theirs!