Reader Rob Jacoby recently sent a question to Paul McBride, the General Aviation News engines expert: I enjoyed reading your article, A simple check to see if your aircraft engine is manufacturing metal, especially the part about oil sampling.
But I have a question: You mention taking the oil sample while the engine has been just shut down (warm) at the midway point.
I usually take my oil analysis samples when the engine is cold. The logic is that all the oil has drained, and the volume removed should be a bit better.
Should I sample midway?
Paul turned the question over the our oils expert Ben Visser, who gives this answer:
The reason we recommend sampling while the engine is warm is that when allowed to cool down, the heavy particles in the oil tend to settle to the bottom and are concentrated in the initial oil out the drain.
If you take the sample on the first oil out, the readings will usually be higher than is representative of the oil change.
By midway, much of the contaminates, like water, have all drained out.
By taking a sample midway through the drain with the oil still warm, you should get a more representative sample and indication of the condition of your engine.
I agree with Ben. The only thing I would add is, “Be consistent in the method you use, for the most accurate readings.”