As Wade correctly stated, most modern 2 stroke oils lubricate well at mixtures of up to 50:1. You can purchase pre-measured 2 stroke oil containers designed for one gallon of gas. They are usually sold in a six-pack of 3 OZ. containers which provides a 40:1 mix. You can purchase the Craftsman brand pre-mix 2 stroke oil at Sears stores in the lawn and garden section. If purchasing the 2 stroke oil by the quart, read the label to ensure it is a 50:1 oil then measure out 3 OZ. for each gallon of gas. That will provide a 40:1 mixture for your weedeater. There are some synthetic oils that claim safe operation of ratios up to 100:1. However, I do not suggest mixing any oil leaner than 50:1.