Hey RJ, for once I'll go the other way
The Snowmobile book says 20:1 but it only goes up to '78. That seems like WAY too much oil. I'll go with RJ at 40:1.
Use a good oil, Arctic Cat, Ski-Doo, and Polaris all sell good oils. Klotz, Amzoil and (damn, I forget, starts with a T) are all excellent synthetic oils. A gallon of good regular oil (AC, SD or Polaris) will run about $20 at a snowmobile shop. I'm using Castol "Go SNOW" right now and it seems real good, its also about $20 a gallon. A gallon covers me for pretty much a whole season, remember you use just over a pint of oil for 5 gallons of gas so a gallon is good for 30 some gallons.
Lots of people use cheap oil from Wal-Mart and the like and thats fine but remember that Scorpion parts aren't getting any easier to find... Don't use use marine 2 stroke oil, it'll want to separate when cold.