I've been using it since '57 or '58. I've tried other products and some of them work better in some situations, but I always have #9 on hand. Probably do 95% of my cleaning with it.
I use it on my nickel revolvers, but I am careful to wipe down the exterior when I finish. I've never noticed any problem, but the possibility of damage may well exsist.
FWIW, I've been told that the formula has changed because the original stuff was a little on the unhealthy side. Don't know if that's true, but it would'nt surprize me.