Boyles law is for gases. For oil, the main factors are Bernouilli's Principle for flow and pressure relationships for change in diameters, the Darcy Weisbach Equation for calculating pressure drop along pipes (which is used to work out pump sizes and pressure settings). Two pipes with the same cross sectional area as a larger pipe will not have the same flow rates due to greater friction because of more pipe wall in contact with the fluid. The shear stresses in the liquid as it flows are annular. The liquid in contact with the pipe wall is stationary whereas the liquid at the centre of the pipe is moving fastest. The gradient of velocity is non linear due to the geometry of the pipe.
These kind of calculations are absolutely critical in larger engineering applications. In sprinkler systems for example, the pipework must be capable of delivering sufficient water at the correct pressure to any part of the system.
No point in me putting much down here - Wikipedia will have plenty of stuff if you want to know more.
