I tell people to be aware that it may have an adverse effect on the wipers. It's not very scientific, but neither are the wipers. They really need all the vac you can get and even then you should carry a can of RainX with you. Personally, I go with electric wipers because I have a bad attitude about vac wipers.
The V-237 PCV Valve that has been tested to work best takes 2 inches of mercury out of about 17-20 available. There is no good way to answer that question other than depends.
