Dormant oil is a stronger petroleum based oil such as 'Volck' dormant oil. It smothers the eggs on dormant tree bark. The box stores sell it. Summer oil aka horticultural oil is cottonseed or other vegetable oil. It works on live soft bodied insects like aphids and mites.

Actually I don't think anyone sells "dormant oil" anymore, it used to be that heavier oils were sold for dormant spraying only but these days all the oil sprays can be used in all seasons. Volck oil for example can be sprayed year-round. So get any good horticultural oil.

I find that for aphids I need to spray at around pink with summer oil, the dormant spray alone does not do it. Even with that I get occasional outbreaks which I need to hit with summer oil. If I see just a few bad tips I simply prune them off.

Scott

Thank you for the info. I bought some horticultural oil at a local nursery.... It says organic on it...