I experimented, after finding an article on the internet, by adding roughly a teaspoon of oil into a Dr Bronner Peppermint soap/ water mix in a spray bottle. I suppose the oil acts as a "surfactant" as well as a contact insecticide ? I sprayed the plant thoroughly, including all leaf undersides ( yep, I am aware of potential transpiration issues by doing that ). The plant gets at least 3 hours of South-facing sun every day.
This was about 3 months back.
See current photo. The leaves still appear glossy ( its an oncid, btw - and can take sun ) and appear undamaged.
My questions - has anyone else tried this ? What were the results ? If the leaves are glossy/oily, does this mean that they are still protected against mites ? I mention mites because I lost a few plants to a mite infestation last year - and now they're gone...