The most important point about oil samples and oil analysis in general is you are looking for any "change" that is taking place. In order to see a "change", each sample must be taken in a consistent and repeatable manner. Oil Sampling can be accomplished in a many ways. However, there are only a few ways in which the sample can be correctly captured.
Each sample point must be analyzed for how best to capture the sample in the most consistent manner. Always begin with the best method and work down the list until you reach a compromise between the best method possible and what the conditions or configuration of the equipment allows. Consistency in taking an oil sample comes down to the person capturing the sample. Proper training goes a long way in reducing the variables in how each sample is actually captured.