Tony and Ivo,
You got me going on this lead vs. no lead question. I have been riding old bikes for ever and currently own nothing newer than 1975, my oldest being a 1967 Dream 305.
I know that when leaded gas was going the way of the Dodo, I read many articles in magazines about this question. My best recollection was that most motorcycles, having alloy heads and being air cooled, use hardened steel inserts for the valve seats. I especially remember that there was no such thing as a 4 cycle Honda which required leaded gas as I owned a couple at the time.
Just in case, I got on a few motorcycle web sites and although it was tough to find anything on this subject as it has been beaten to death in the past, everything I did find confirmed my recollection.
I guess I will continue to run no lead, my 1971 Honda 500/4 has been running on it for 4 or 5 years for 20+ K miles, it now has 42K, with no apparent ill effects. i.e. no sudden valve clearance changes.