I went through an ordeal of notchy shifting on a prior car to the point where I invested a lot of time in oil research that eventually led to a solution. Here's the quick summary, perhaps it will be of help:
Our gearboxes use synchros to aid shifting so we don't have to double-clutch, like all modern manuals. In my prior car the synchros (there is at least one, sometimes more on a gear) were not doing a good job, even though they were in good working order. The synchros need friction to spin and do their job, too much lubrication and there is not enough friction for them to do their job. Too little lubrication (too much friction) and they eat themselves up and need to be replaced.
Basically, a transmission builder told me that there is variation in transmissions, which means there is a different level of lubrication required for the synchros to do their job -- more like a range of lubrication where the synchros work well. Temperature affects this range, so if you are near the edge of the range your transmission may shift better warm or cold. For most transmissions, the recommended fluid provides the necessary friction and it shifts well. As the oil ages the lubrication changes, replacing the fluid at this point may fix the issue. However, as transmissions age, the friction requirement may also change (so I was told).
In my case, the OEM fluid made shifting notchy and after trying several different fluids it still wasn't quite right, but some definitely made it better. Basically, the fluids were close, but not quite right for my transmission. The transmission guru told me that most of the fluids have a friction modifier in them to help hit this perfect balance of lubrication/friction; in fact, many supplier have a range of fluids meant to serve the varying needs of transmissions. Sometimes, the transmission falls between these and none are perfect. My transmission was like this, so he told me the solution was to start with fluid that has no modifier
Then gradually add modifier one ounce at a time, drive for a while and add more if necessary.
Surprisingly this worked. I found a ratio of modifier to fluid that worked best and tweaked it over time as the transmission got older.
The usual disclaimers apply: Do at your own risk, make sure you pick fluid/modifiers compatible with your transmission; feel free to use this as a starting point in your own research, etc. Hope it helps someone.
BTW: my 6MT shifts fine and I've never felt the need to touch the fluid.