Sorry to hear about your 'trouble'.
Unfortunately I have the same problem and have also tried quite a few 'recommended' oils without any significant improvements. Oddly enough I have noticed that when it's a bit colder it gets a bit easier to change from 1st to 2nd. I've read about that elsewhere on the net and it is suggested that it's because the oil gets thicker and stickier and 'grabs' the sync rings better. A thicker oil could help if that's true - which I also read somewhere...

I will try a few things later this year (when it gets warmer):
1. Check the gear-shift lever housing bushes. They are made of Polyurethane and if they are worn they could perhaps affect the alignment of the whole assembly in a bad way.
2. Check for wear or dirt in the gear stick 'main bushing'.
3. Check for rust/dirt on the main shaft sticking out from the gear box - then grease it.
4. Fit a 'Sports Shift' kit as recommended to improve gear changing on the MT75.

Of course I will publish all findings and results here on TM.

-99 4/4, 1.8 Zetec, Indigo Blue, 2-seat, Suplex RS
-04 Citroen Pluriel, 1.6 Sensodrive, Met Orange