How can you calculate air resistances at different speeds? I've read that at 50mph air resistance to an average car is the equivalent of driving through water and at 80mph it's the equivalent of driving through oil. 
I can't find any references online to back up these figures. Is there a relatively simple way to explain the calculation to verify these figures, or are they incorrect?
 A: If I understand your question correctly, one aspect that you seem to be asking about is the relationship between and object's speed and the associated air resistance (as in your title) - which is often referred to as 'drag'.
The Physics Hypertextbook chapter Aerodynamic Drag, generally relates drag, defined as:

The force on an object that resists its motion through a fluid is called drag ($R$). When the fluid is a gas like air, it is called aerodynamic drag (or air resistance).

as being proportional to the square of the object's velocity ($v$) as:
$$R \propto v^2$$
So, you can see that as you get fast, the effect of drag increases faster.
This relationship is from the drag formula:
$$R = 0.5\rho CAv^2$$
(where $\rho$ is the fluid density, $C$ is the drag coefficient and $A$ is the surface area affected)
A: The resistance to flow, can be "naively" said as proportional to the velocity. It is in fact proportional to the integral over the area times the velocity gradient. The constant of proportionality turns out to be viscosity for Newtonian fluids. The exact calculations are tedious and depends on the nature of the problem.
To answer your question, you could say that it is proportional to the ratio of $\mu v$ where $\mu$ is the viscosity of the fluid and $v$ is the relative velocity under consideration. Now $\mu$ is an experimentally determined quantity and it varies between what "oil" are you referring to.
