Another suggestion is to find your local chainsaw dealer, or small engine repairman. Loggers are especially particular about what they put in the Stihls and Husquvarna's.

With folks coming around for demo rides, I use about 2 gallons of 50:1 mix a week. But in the slow times, I never let pre-mixed fuel sit around for longer than 3 weeks, put unused oil/gas in the car and mix up a fresh batch.

I would just guess that rule of thumb would be doubly important when you are mixing 20:1.