Hi Miz
Any new bulk oil shipments are sampled and sent to an independent analysis lab for FTIR and full spectrographic analysis. This is used as the base line for the batch. The load is isolated until results are returned and evaluated. Results can be compared to published product data sheets also.
We encountered a cross contamination issue once that was caused by an improper flush procedure, allowing a previous oil delivery to remain in the tanker trucks hose and pump. The sample drawn was darker in color and the viscosity - while still in ISO range - was at the higher limit. Further investigation revealed that it would take at least 25 gallons of oil to flush the tankers system properly to prevent cross contamination.