Seem to be a regular event that someone posts a problem with GP gearboxes.

Has anybody any advice/experience with aditives like molyslip or similar to keep the gearbox working sweeter/longer?

I believe that the first step is to change the ones that Fiat recommends and are completely crap. M1, Redline, Torco, Fuchs, Amsoil are top alternatives.

Be careful because not all the additives are compatible with each valvoline. They might do it worse so in my opinion choose a decent valvoline instead of searching for additives.