There is a lot of heat transfer from the engine into the clutch slave cylinder. I find if you don't bleed the clutch at least once during a race weekend, it starts to degrade fast. I see it also on road bikes once the fluid has about 5000 km or a year on it, or operates in hot conditions.
I use Putoline synthetic brake fluid now which works well, or RBF 600 on the race bikes. Bleeding from the bottom nipple to the top nipple, then a last two pumps on the bottom before wicking out any residual fluid and the clutch is good to go.
On the old 748RS we used to have clutch lever fade due to heat on the first generation of the unrebuildable slave cylinders (from 2001 on) and I ended up milling up heat-resistant spacers to stagger the slave cylinder out a bit (3mm) from direct contact with the engine which cured the problem.
Better-quality billet slave cylinders also reduce the problem. Not too sure why, but I suspect better heat dissipation and not enough cast material acting as a latent heat sink.
