Can anyone offer advice about the application of horticultural oil on fruit trees and rose bushes? I've read the directions but am still rather uncertain.

LindaLou, it helps if you put your climate zone on your signature so we know where you are.

In CA I spray horticultural oil once a year, in Jan, after pruning. But the light oil can be used at other times too, just not when the fruit trees are blooming so it doesn't affect the bees. I use it on: roses, bay tree, apricot tree, brugmansia, and citrus. Try to time the spraying so it doesn't rain within 48 hours and wash it away. It rains so seldom here that's no problem.

Mix per package directions and spray to the point of runoff on a day when the temperature is above freezing and will remain above freezing (several hours) until the oil mixture dries.