The instructions of my L Series say to clean with a soft cotton cloth. I normally take that as meaning the bottom of my shirt.

Other than that I I just use a dry micfofibre cloth, I have some solution back at home, but I couldn't tell you what it is as I've had it so long.

I'm not one for using UV/protection filters either, as I don't shoot with the lens hood most of the time I find it causes loads of problems. I'm fully ensured and the excess is much less than the cost of a decent filter anyway.

Cheers

Cris