I use a similar generic product Acidified Isopropynol that pre-dates
Rain-X by many many years. In fact, I think Rain-X is the same
generic product only repackaged in consumer sizes and sold under that
I lived in St. Louis where it gets mighty cold, and the generic
product is used on most commercial aircraft as well and never had
problems from using it.
You will get windshield wiper chatter as it wears off though unless
you also use dry silicone on the wiper blades.