The best viili is made with raw milk which has its own name in Finnish: tinkimaito.
In this article in the paper Maaseudun tulevaisuus they argue the case for saving the “tinkimaito” (raw milk bought straight from the farm) that Finnish central government is now threatening to forbid. The sale of tinkimaito has actually risen a lot over the last few years due to the fact that lactose-intolerant people can still drink it. The raw milk is also known for many other heath benefits:
“Few people are aware that clean, raw milk from grass-fed cows was actually used as a medicine in the early part of the last century. That’s right. Milk straight from the udder, a sort of “stem cell” of foods, was used as medicine to treat, and frequently cure some serious chronic diseases. From the time of Hippocrates to until just after World War II, this “white blood” nourished and healed uncounted millions.” Read more about the benefits here: http://www.raw-milk-facts.com/raw_milk_health_benefits.html
Download the PDF of the article here (unfortunately in Finnish only)