I usually mix either 1/2 gallon or 1 gallon at a time. I have had the syringe style measuring device, but it was messy after use, as not all the oil gets pumped out of the syringe & attached hose, and it then leaks out of the tube over time. I kept it in a plastic bag, but it just wasn't all that convenient for me. The black markings on the side of the syringe also came off of mine easily.
What I use is a Tupperware midget. It has lines molded into the cup at 1, 1.5 and 2 ounces. I usually mix my fuel at around 42:1, so I add 1.5 oz. oil into 1/2 gallon of gasoline. If I am mixing one gallon, I pour twice from the cup to get the 3 oz. necessary. The top can then be snapped on the Tupperware cup to keep the residual oil from leaking out, and to keep the inside of the cup clean.
For different ratios, like 50:1, which would need 1.28 oz. oil for 1/2 gallon of fuel, it is easy to estimate by filling the cup to just a little more than half way between the 1 and 1.5 oz. markings.
Here is an image of a Tupperware midget...