A point to bear in mind is that we are talking about lubricants for vehicles over 30 years old.
Lubricants have moved on and when we used to change mineral engine oils every 6000 miles modern oils will be specified at 12,000 and 18,000 miles.
Gearbox oils have likewise improved. The ZC90 was recommended to be replaced every x thousand miles / y years. Modern gearbox oils (I think the likes of 75 Synth) are fit for life where life is 100,000 or more miles. Don't quote me on these figures as I've not checked for a few years on the exact renewal specifics/recommendations.
What is true is that more modern oils (especially synthetics) are generally better than the older mineral oils in both life and performance. Traditional manual gearbox gears, synchros, diffs etc. have not radically changed in 30, 50 or more years. What has changed is the seal material composition and some older oil seals/others are not necessarily as resilient to the more modern synthetic oils.
There are reports of some engines that used to run on basic mineral 20W/50 or 10W/40 oils being refilled with 5W/30, 0W/30 and similar fully synthetics oils and the engines later later leaked like sieves.
I have a 130TC, has 90K miles on the clock and has done over 6000 miles of motor sport related stresses. All done with ZC90 gearbox oil. This is enough for me to say that ZC90 works. I could possibly improve with ZC75 Synth but whilst I can still obtain ZC90 (or similar) then prudence suggests stick with it until otherwise forced to deviate.