Was changing out the drive oil for winterization yesterday on my 34' cruiser. Twin bravo 3 drives. One drive, when turning the props by hand while in neutral is very hard, and gearish sounding. The other drive, smooth and quiet. This has been this way all season, the drive that turns hard was supposedly rebuilt, and I picked it up this spring. When dropping the oil, the hard to turn drive oil looked pretty clean, and still green. The smooth drive oil looked like used motor oil. Was pretty surprised to see the difference. Not sure if the dark oil drive was getting hot for some reason? I had fresh fluid in both before the season.

Any thoughts?

green fluid with hard to turn drive by hand

dark oil with smooth turning drive by hand

Do you see any metal on the magnets of the drain plugs?
as long as you don't see any metal other then a very fine dust....or water on the oil i wouldn't much worry about it.
The re-builder may have set it up on the tighter side of specs and it just needs some time to break in and loosen up....
I had a pair of TRS drives rebuild some 20yrs ago and one was always tighter then the other when turning by hand....didn't have any problems for 5yrs of running them and sold the boat and the new owner didn't have any issues for another two years before he sold it...
As for the black oil....unless its getting hot id say its also not a big deal unless again you are seeing metal on the drain plug.
You will never get 100% of the old oil out so that in itself will turn the oil darker sooner.....but id also look for white calcification on the tops of the drives where there is very little cooling going on when the boats on plane....that's where it will be running the hottest.....maybe look at investing in some drive showers to help cooling.

Make sure your dark oil reservoir is clean
maybe the dark comes from the black **** sitting on the bottom of the reservoir

My opinion is the drive is getting to hot. Did the dark lube drive have Merc lube or something else?
Turning the props in neutral you will hear and feel the lower gears. It won't be loud but can hear

When my BIII started to blacken the gear lube (Amsoil Marine and I ran a drive shower) it ended up being the gears starting to pit and shed material. Magnet looked fine. It was like it was powdering the metal shedding off the gears. Lowers were the worst. The uppers just showed some pitting.