I used molyslip in mine because of the 2nd gear issue. I actually used two tubes. Made no difference at all.
I then replaced the new fluid with 75w140 - that made it slightly worse. Expensive experiment.
What did remedy it, and I mean, it actually cured the problem, was 2:1 of 75w90/5w30
Didn't cure it straight away, it took a few weeks, and now all gears are smooth. I can only conclude that as the boxes wear, they become averse to 75w90. Too thick.
I was planning to rebuild the box anyway, hence my experiments, but now it's fixed, I'm happy