Mix fuel with oil by first determining the appropriate fuel to oil ratio for your engine, and then mixing the oil and fuel in an empty fuel container. Do not use a container with any residual fuel in it, even if it is the same fuel your engine requires, as doing so interferes with the correct mix ratio.
The key step in mixing fuel and oil is determining the correct fuel ratio for the engine. The majority of small engine equipment uses either a 50:1 or 32:1 fuel-to-oil ratio. The correct fuel ratio is always printed in the equipment's owner's manual. Some manufacturers also print or stamp the correct ratio directly onto the engine or gas cap. If you do not have the owner's manual and the ratio is not printed on the engine, contact the manufacturer for the correct ratio.
Once you have the correct ratio, measure the appropriate amounts of fuel and oil before mixing. For example, if your engine uses a 50:1 ratio, pour 2.6 ounces of oil into the container, and then add 1 gallon of fuel. When mixing the fuel and oil, always pour the oil into the empty fuel container first. When you add the fuel, the two liquids mix naturally, eliminating the need to stir the contents or shake the container. Only mix as much fuel as you need for a few days' work, as the two liquids eventually separate, which can cause problems in the engine.