A major concern for anyone who owns a two-stroke engine such as a lawnmower or an outboard motor is the proper ration for oil to gas, as a two-stroke engine -- unlike a car -- does not have a separate system for distributing oil and it must be contained in the fuel to keep the engine properly lubricated. Getting this ratio right for your outboard motor will make it last longer and run cleaner.
The perfect mixture for two-stroke engine fuel is commonly known as "the golden ratio." This refers to the amount of oil needed in relation to the amount of gasoline. It is surprising to learn that the perfect ratio is 50-1 -- 50 parts gas to one part oil. This can seem like it is a tricky bit of math when you're looking at a gas can. Generally, 5 gallons of gas will take 1/2 quart of oil, 2 1/2 gallons will take 1/4 of a quart and so forth.
For your convenience and to make sure you have the proper ratios, pre-mix your fuel. If you have a 2-gallon gas container, fill it up, add 1/4 of a bottle of oil and you're done. This will save you from guesswork when filling a tank that may be half-full already. Eliminating the guesswork of how much to add may save your engine and make it much more convenient when you are out boating. You don't want to fill up and then be caught without the proper amount of oil, so mix it before hand and carry your own fuel mix with you.
Once you have added enough oil to the gas, shake the gas can gently to make sure that it mixes well. One gallon of gas is 3629gr, and you can add about 88.7ml. of oil to a single gallon. Use the see-through measurement indicator on the side of the oil bottle to make sure you are adding the right amount. Most boaters make sure they have enough by carrying two 1-gallon gasoline cans with the oil already mixed in.
Choosing the oil for your engine is not as simple as stopping in at the local convenience store or bait shop and picking up a bottle of motor oil off the shelf. Some engine brands, such as Mercury, recommend that you use a specific oil that has been tailored to accommodate their engines. For Mercury, this brand is OptiMax/DFI synthetic blend, which is available at outboard motor shops and marina stores. Check your engine's owner's manual to see what oil it recommends or ask at your local marina store.