Often, dormant oils are relatively heavy oils designed only for use on dormant plants. Purity ratings of dormant oils may be relatively low. Summer, superior or supreme oils are products of lower viscosity and higher purity than dormant oils. Many of these lighter oils are capable of being used on the foliage of tolerant plants without causing plant injury. Spray oils are almost always formulated with an emulsifier to allow mixing with water.
Emulsifiers also affect droplet size of the oil spray.