For a 50:1 oil-to-gas ratio, use 2.6 ounces of oil per gallon of gasoline. In other words, for 2 gallons of gasoline, one would need 5.2 ounces of oil.
There are 128 fluid ounces in one gallon. To calculate the appropriate amount of oil for 1 gallon of gasoline, divide 128 by 50 for the 50:1 ratio, which equals 2.56. Rounding up that number to 2.6 will provide the appropriate quantity of oil for the ratio.
This same formula can be used for other ratios. For example, a 40:1 ratio requires 3.2 ounces of oil because 128 divided by 40 is 3.2.