Michael,
if your Sidewinder is 20%N - 12%O this would mean that you have a gallon of the Sidewinder Race blend.
if its the Sidewinder backyard basher than the 20%N comes with 16 and 18% oil.
one thing you need to do for certain is that you cannot put in just any kind of oil, you need to get your hands on the proper synthetic oil or some proper Castor oil (not just any Castor, it won't do).
You can always buy yourself another gallon of fuel and sell the one you have.
on another note, Mike Hallam also sell WildCat Fuel Promix, the promix comes in 20, 25, 35, 45, and 65% Nitro, it has 18%oil (95% synthetic and 5% degummed racing castor).
that is the fuel I have been using for 3 years, great fuel.
Dan.