Yes, horticultural oils can do some leaf damage if applied at temperatures at or above 85-F. However, a citrus tree should NEVER be sprayed during the heat of the day with anything whatsoever, I would certainly not call today's HO a heavy oil. Spray early in the morning, later in the evening, or on cloudy days. A good brand of HO is Ultra Pure HO which is used at 80-grams per two gallons. Works great.