Q.Pittosporum losing leaves
I have three 4 year old pittosporum trees. They were growing well and bushy until I discovered sooty mould on them and they had black scale. Ive since treated them with white oil which seems to have fixed the issue but the trees have now lost a lot of their leaves lower down and are sparce. My Q is will they generate regrowth and bush up with new leaves again? Should i prune them which will make them even sparcer or will this help? See photos.

Certified GKH Gardening Expert
Was this actual mineral oil, or was this a home meade soap oil? Actual white oil will not cause these symptoms, but soap can if it is not diluted enough.
Another issue could be that an insect is not to blame for the damage. Fungal issues are very common and can look like scale sometimes. Insecticides will not be effective against infections of this nature.
Can you include a photo? This will help me to see what is going on, exactly. Then I can recommend the best remedy from that point.