Gary, I have used the Mother's brand convertible top cleaner on my white top and I was pleased with the results.
My friend's dad swears by household "Comet" cleaner. Thats right, the powdered stuff you clean your toilet with. He's used it on his 61 Buick since he bought it new. And he still has the original top on it from 61!
I used Comet on my white top about once a year for a really good cleaning. Didn't have any ill effects in the couple of years I had it.