The "glass" on my turn-of-the-century L2600 is not glass at all, but some kind of plexi.
I had the same problem. I used to put a little spit on it, and that would sometimes help clear things up. Eventually, I had the dealer replace it (when it was in for other service). The dealer assured me that it was a "simple" procedure to replace it, but I wasn't so sure about that at the time. I *think* you pry out the old one and pop in the new one (with some sealant). It's not a pressurized system at the point of the site glass.
I decided that next time mine gets to looking hazy, I will try some polishing compound like what is used to "restore" headlights.
I think there is some potential problem by overfilling the hydraulics at the case there, so I would not just go filling it until you can see it.
Also, it is also not an uncommon scenario to have the fluid level full, and not appreciate the white background / fill level...
