From the paper: "The thickness of oil required to take the life out of the camphor movements lies between one and two millionths of a millimetre, and may be es

From the paper: "The thickness of oil required to take the life out of the camphor movements lies between one and two millionths of a millimetre, and may be estimated with some precision at 1’6 micromillimetre." Looks like a primary guess to me, even if the table lists more data points.