think of it in "parts of one component" to "parts of the other component" then there is no confusion about weight or volume...... if one "part" is one ounce of gas or oil, then a 30:1 ratio will be 30 parts gas to one part oil. or 30 ounces gas to one ounce oil. this can applies whether you are measuring ounces, pounds, tons, quarts, gallons or swimming pools full. as long as you work the ratio in the same weight "parts" or the same volume for both "parts", the mix will be correct.
30 parts gas to 1 part oil
30 ounces gas to one ounce oil
30 gallons gas to one gallon oil
30 pounds gas to one pound oil
etc, etc,.....all the same ratio.