Can’t comment on how correct any of below is but from what I've read on here & been told on my CBT you shouldn't/should use:-
Don’t use Fairy liquid as it contains salt, which will scratch and bloom your visor through time.
Don’t use any “cleaning products” even though they are described as helmet safe as there has been cases of people cutting the helmet apart & finding even just the vapours from them having degraded / dissolved the polystyrene around areas such as the vents.
Use water & kitchen towels! and if it's really stuck on, leave it with the wet towel on it and have a beer. When you come back to it will be softer and easier to remove.
Legal Disclaimer: - Of course don't drink after riding, Neither my instructor who told me this technique, nor this site condones irresponsible behaviour.
Always drink responsibly (and in large quantitys).
See DP, I was listening, and even remembered