I have a gallon of fuel 20% nitro 16% oil, I wanted to increase the oil content, how many ounces of oil do I have to add to make my fuel 18 to 20% oil ? By the way I have a bottle of Klotz synthetic 2 cycle racing oil.

Your replies are welcomed

You need to add 3 ounces of oil.
Right now you have 20% Nitro, 16% oil and 64% Methanol.
After adding 3 ounces you will have 19.5% Nitro, 18% oil and 62.5% Methanol.
If you want to play with numbers a bit more go to
http://www.nitrorc.com/fuelws/
or if you want a calculator that would help you in making your own fuel from scratch then go to http://www.angelfire.com/in3/aeronuts/fuelcalc2.html

Hope this helps