We trad down to minus 6 and I think the coldest we worked last winter was down to minus 8 but it depends on many things. Shadow, sun etc. We only trad downsatirs for a reduced price when snow is on the ground. Got that idea off Dazmond and Matt Bateman a few years ago. We drop the price by about 30% and custies seem happy with that.
Cold water with screen wash in. No soap. Try it if you need to, nothing to lose.