Great job on explaining Principles of Fluid Dynamics. Are you sure you're no a teacher or student under some engineering program?
Yes, fluid requires a specific volume to generate proper balanced compression, or at least, initiate pressurization. Just like air, you need a specific volume to either compress or expand to obtain a constant value. Water is the same. Although water cannot be compressed or expanded (liquid and not gas or solid), there is a specific volume that is required to initiate an equal pressurization for a containment unit, based on the contaiment (the Bathmate total volume) and the occupying body (your penis). There must be at least 10% volume space of non-expansion (after your penis expanded to intended size under vacuum pressure) to equalize the pressure. If the occupying body continues to expand, pressure cannot be equalize continuously in the containment vessel. This is why the head space is required, including any surround extra room.
As for warm oil, it's basically like water. Oil, depending on the oil, can be great use to you, not only acting as a longer warming agent, but also acting as a lubricant to your skin to prevent chaffing. However, most oils are compressible liquids. Denser oil than water (the one that actually sink and not float) can be used, such as mix of silicone and additives, petroleum byproducts, and gels. But, it's no longer safe using them. Yeah, you're going down a very deep rabbit hole brother.
But hell... Even Einstein worked at patent office to gain some knowledge. 
