Hu-974821871, yellowing and dropping of leaves can occur for more than one reason. Desert Roses are highly sensitive to big changes in temperature, over-watering, over-fertilizing and various munching pests. In my experience, spraying the leaves with any kind of oil based product, will damage the leaves and quickly lead to its demise. When I do spray with an oil based product to kill aphids or mealy bugs, I spray down the plant's leaves within a couple of minutes to rinse off the oil to prevent leaf damage. So far this seems to work for me.

You said that you are experiencing drought conditions currently. Depending upon the size of your plant and its container, if your temperatures are quite high, I would water your plants more frequently. Desert Roses once upon a time lived in wetter tropical climates and eventually evolved to withstand drier conditions with climate change. Living in Hawaii, I have learned that my plants need more frequent watering when the summer temperatures are very high. As the days shorten and the day-time temperatures become more moderate, I can reduce the watering slightly. My plants actually put on a growth spurt in the last month with the lower temperatures. There is a university in the American state of Arizona which does research on Desert Roses and they claim to water their plants daily when the temperatures are extremely high. You may have to experiment to see what watering schedule suits your plants best for the conditions you are offering them.

As far as your potting medium is concerned, I would highly recommend that you do not use sand of any kind. Sand tends to become very dense when wet and prevents quick water drainage. This will lead to rotting of the roots. You would be far better off using a coarse grade of perlite and fine gravel to give you ideal drainage. Instead of gravel, I use fine lava cinder because it is cheap and readily available in my stores. Cactus and succulent soil is perfect for your plants so continue to use that as the main component for your potting mix. Good luck to you with your hobby.