According to CSGNetwork.com, most marine requirements, such as personal water craft (PWC), jet skis and outboards, are 50:1, gas to oil ratio.
For 5 gallons of gas, with a 50:1 ratio, the amount of oil required is 12.8 US ounces (100 ml). Experts agree that it is better to err toward the side of too much oil, rather than not enough. Typical break in period ratios are about twice the normal; 50:1 is generally 25:1 during break in. Do NOT add oil to the fuel of 4 cycle marine engines.
If you are mixing your oil and gas in the water, use great care to avoid spills that will ultimately pollute the water. Also check the two cycle engine requirements for the body of water you are planning to use.
You can use this Marine Gas Oil Mixture Ratio Calculator from CSGNetwork.com.
It's easy to keep your outboard in tip-top shape, even if you're not a mechanic. Preventive boat motor maintenance facilitates safe boating and can keep your motor reliable for a long time. Learn about motor maintenance safety rules, boating safety tips, boater checklists and more at Discoverboating.com.