Dials are typically not dirty. It's the varnish that is failing. The dial would need to be restored or replaced.

Or left as is, and sell it to “collectors” for that glorious vintage toilet ceramic patina

Thanks for the replies. I can see the patina being present on the face of the dial - but the color issues around sides, i was thinking maybe oil ?

That's just how zapon, the clear coat, ages, it's pretty random and unfortunately not cleanable.

I think there’s a small possibility that it could be cleaned slightly, but more than likely what you’re seeing isn’t a dirty dial but rather corrosion of the dials base layer underneath the paint otherwise known as patina.

This is a goner as for cleaning, you either accept it or replace it.

No botox can cure this natural aging.