It depends upon the manufacturer and age of your o/b engine. Most older engines take a 50:1 mixture of gasolene to oil, so 64 oz/gal times 6 gals = 384 oz. Divide that by 50 and the result is approx 8 oz of 2 cycle oil. Newer engines often call for a 100:1 mixture, so it would be 4 oz of oil for 6 gals of gasolene. Really old or low technology engines (e.g. a British Seagull), might take as much as 8 oz/gal.