The successful implementation of a crude oil custody transfer sampling system is a key component to achieving a desired measurement uncertainty for a crude oil metering station or loading/unloading point. Our analysis of thousands of installations worldwide provides practical examples of how operators can be confident that when they install a sampling system, it will deliver the uncertainty that they need to meet the overall custody transfer requirements. Crude oil sampling for custody transfer becomes more challenging as production flow rates decline, oil fields mature, and water cut content increases. It is therefore important that the performance of a sampling system is evaluated on a regular basis and that any limitations are identified. Any risk of change in performance or measurement uncertainty can then be prioritized or minimized. International standards and contracts determine the allowable uncertainty for net oil (oil minus water) for custody transfer/allocation. For accurate allocation of the sources of crude oil or the application of tax tariffs, fluids must be measured prior to being comingled. Automatic crude oil sampling can be challenging because it can require high-energy mixing with low power consumption and negligible pressure loss to overcome stratification and allow representative sampling. The certification, or "proving", of the sampling system provides a basis for establishing and verifying the system's true uncertainty at worst case conditions. There is an abundance of computational fluid dynamics (CFD) simulations and studies on crude oil (and water) mixing. However, these are abstract because of the uncertainty in where the water is located and how it may be dispersed at the boundary conditions of any simulation. To provide more robust simulations, we expanded on the established methods by combining simulation data with known theoretical calculations and engineering laboratory test data as well as hundreds of certifications (proving) results from around the world. Automatic sampling systems using dynamic mixing technology delivers a unique solution that enables operators to minimize the quality measurement uncertainty, improve overall balance, and reduce financial loss (and unaccounted for) in custody transfer quality measurements.