40:1 is probably the ultimate safe mix. I use it because my wankle Panther wants 40:1. While my Tigre wants 50:1 it'll still be happy with the extra oil and its only a little hard on plugs.
50:1 was recommended by Arctic Cat for ALL Spirit motors. Use a good quality oil (like Arctic Cat oil) and feel secure. The best way to make sure you're getting your ratio correct is to use a ratio rite cup. They're cheap and last forever...
Otherwise put about half a quart of oil into 5 gallons of gas. 5 gallons = 20 quarts. 1/2 quart of oil : 20 quarts of gas = 40:1 mixture.
Raising sleds from the dead in north central MA.
'70 Ski-Doo Olympique 12/3 - The Yellow Bullet
'71 Arctic Cat Panther 303 - Wankle Panther
'78 Arctic Cat Pantera 5000 - Back from the dead!
'79 Polaris Cobra 340 - Back from the dead!
'95 Ski-Doo Grand Touring SE 670