airflow indication
I believe he is talking about using oil to visually determine the airflow over an object. I would not use oil myself, but the idea is use a thick liquid that will leave a trace after flying. Apply, fly, land and check. The fluid will flow back and around an object leaving streamer or streaks. From this you see how the air is flowing. In the case of the gear leg the fluid should flow back the same on both top and bottom. If it is angled too much in one direction the airflow and the fluid will detach also, showing an indication by not marks. You can use colored dyes to do the trick . Good luck