With a few reasonable assumptions, the flow of liquids is not as complicated as it may seem. Most liquids compress so little that they may be treated as incompressible in real world calculations. Most of the oil base stocks are close to newtonian fluids, viscosity changing little with shear. A completely formulated motor oil is not quite as close, but still often can be treated as such in the real world.
Flow through a pipe is linear with pressure, length, and viscosity. Twice the pressure, or half the viscosity or length, gives twice the flow. The flow varies with the 4'th power of the radius for laminar flow or the 5'th power for turbulent flow. If you lay ahold of enough data, you can calculate the Reynolds number to determine which to use. Either way, the size of the passage way has a enormous effect on flow, about 16 or 32 times more flow for twice as big of a pipe.
With the above assumptions, you can understand much of what is going on, and solve many real world problems.