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!

 

Well what do you know. Been exactly 5 years since the last update.
But now I have a FINAL update, because I fixed it permanently!
Might be helpful for many others that have a stiffer gear change.
Maybe even that notorious second gear?
So what was the main problem?
The internal linkage.
The internal linkage to the gear forks sits in a dedicated part on the side of the gearbox.
First picture on my spare gearbox, the cover that has 10 nuts to hold it.
The second picture is where I have removed it from my car. I had to remove the left rear wheel, aero covers, gearbox bracing bar and I even had to disconnect the exhaust from the last mufflers to lower it a bit so there was enough space to remove the linkage cover. The system inside the cover stays in the cover, only things that can fall out are two detent parts on the edge of the gearbox. One spring, and one dowel pin like piece. But normally they will not fall out, the gear box oil make them stick in their places.
When I had removed it, I noticed that the stainless rod inside that connects to the outer linkage was very, very hard to move. Sideways could be done by hand, but up and down only by brute force. (I had to use a plastic hammer for that.)
Definitely not good.
Then I took it apart by disconnecting the two springs, and the 17mm bolt. Taking the stainless rod out was the hardest part, but by hooking it on a small rod in the hole, I could get it out by gently tapping the cover.
So what was wrong? Very simple, the dark "cherry" connected rings are made of regular steel.
And they had developed a tiny bit of rust because the car had stood for a few years in a collection. The linkage is not (completely) under oil level it seems and sits partly in air. Because it has a moving tolerance of almost nothing, the little rust had jammed the stainless rod.
After cleaning everything with sanding paper 1000 and 1500 , and using a lot of Clevite lube, it all felt positively smooth . I connected the bolt again (with loctite), and I could move the rod by hand in all directions.
Now the gear change feels literally like new again, and all is well.
So if your car has no clutch or syncro problem, but still feels like you need to much force to move the lever, there is a very good chance you have the same problem.



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