The frequency (<500 KHz) used for such sensors propagates through air just fine. The oil you may be thinking about for ultrasonic imagining is to prevent reflection at the skin. They also operate at considerably higher frequencies usually.
The units I have seen are generally for locating hard solids, like plastics and metals. I am not sure of the reflective properties of skin and how much it varies from person to person or location to location on the body. So, while 0.5 mm precision may be attained with solids, I suspect with soft tissues it may be a problem. Why do you need such high precision?
The spot size for the ultrasonic sensors is shown in the datasheets. If the geometry changes from normal to 45°, I suspect the precision (however it is defined) will also suffer.
John