I quoted both of you because you both have basically the same concerns.
Being a person that was in the trans business for years and especially with older manuals I will tell you what was done as accepted practice as far as lubes and syncros goes.
There is truth to the statement that synthetics can interfere with the intended function of syncro rings, but that fact is also open to preferences in the trans function under intended usage.
We would start with the basic recommended fluids, like 30wt motor oil for example.
If the trans was hard to shift or didn't like downshifting without what was called a crunch going into gear, we would swap it out for automatic trans fluid because it was thinner which aided in shifter movement ease and also let the syncros grab the next gear quicker and alleviate gear crunch on shifts.
Point is, it's actually a bit less important how thick or thin or synthetic the fluid used is as much as how the trans works for you, as it is more of a viscosity balance than a lube quality question.
These older transmissions would run just fine on beaver pelt oil and be fine as far as lube goes.
In cold area's 75/90 will feel like trying to shift through cold honey, and because of it's viscosity could even cause gear crunching in cold weather due to the syncros not being able to get down through the lube and stop the upcoming gear .
In the deserts or just plain hot weather that same fluid might work just fine, but I would hesitate to go as thin as ATF in hot climates.
