CSU Extension Historically a variety of oils were used for insect control including vegetable and fish derived oils. At present, all spray oils used for insect control on woody plants are petroleum-derived specialty oils. These are highly refined so as to limit their potential to cause plant injury. Several brands are currently marketed and can be compared and categorized primarily by their viscosity and purity.
Viscosity refers to the body or flowability of an oil.
It is based on the time required for a uniform volume of the oil to pass through a standard, small opening. Heaviness of oils is also measured by its volatility or distillation range of temperatures. Less volatile oils are usually more insecticidal but also can be more damaging to the plant. This U.R. or U.S.R. rating is typically in the range of 90 percent to 99 percent. Higher percentage ratings indicate products of greatest plant safety.