Gearbox/clutch problem, won't shift into some gears

Update! Hopefully final.
I changed the green gearbox oil (I think it was Redline: it was green and smelled awful of sulphur, which I know Redline does) for some 4.5 litre of clean Castrol 75W90.
There was no metallic swirl in the old oil, nor much deposit on the magnetic plug, so that was good to see!
I did notice however that the oil that came out of the cooler/radiator, seemed darker green than the oil that came out of the box itself.

Took it for a drive, with a lot of up and down shifting (second works perfectly from cold by the way) . After 10 minutes it started to go a bit lighter, and after half an hour or so it almost shifted normally into 3 and 4.
It still is not 100%, but if it had been like this from the start, I certainly would not have bought a spare gearbox, and would have not considered it a problem
So I am confident that it will improve further with use.
In the meantime the premium oil arrived (took more than 2 weeks, I guess they had to catch the dinosours first) , so that will go in after 500 km or so.

Anyway, that green stuff that originally was in the gearbox was very sticky, which might be good for the synchros, but maybe not for the detents when the car stands still for a rather long period.
This "simple" synthetic Castrol at least has freed the detents a bit of the stickyness.
Thanks all for your advise, and for me the lesson learned here is that standing still for a long period of time can really give problems in ways you do not expect.
So drive your cars!