Which oil was used by Robert A. Millikan in his oil drop experiment?

The name, as far as I can find, is "highest grade of clock-oil" as mentioned in his 1913 paper (page 111, last line). The density of this oil at $23\, {}^\circ \mathrm C$ is $0.9199$, but the units aren't mentioned.

EDIT: I went to look up an earlier work (1911) and found two other oils of interest that were used, namely "cleaned gas engine oil" of density $0.9041$ and the more volatile mineral oil (machine oil) of density $0.8960$, both measured at $25\, {}^\circ \mathrm C$. I believe these are the oils the Wikipedia entry alluded to.

Source: Wikipedia Oil Drop Experiment

A fine mist of oil droplets was sprayed into a chamber above the plates. The oil was of a type usually used in vacuum apparatus and was chosen because it had an extremely low vapour pressure. Ordinary oil would evaporate under the heat of the light source causing the mass of the oil drop to change over the course of the experiment. Some oil drops became electrically charged through friction with the nozzle as they were sprayed. Alternatively, charging could be brought about by including an ionising radiation source (such as anX-ray tube). The droplets entered the space between the plates and, because they were charged, could be made to rise and fall by changing the voltage across the plates.

Paraffin oil has a vapor pressure of 0.5 kPa, as a benchmark. I think the original paper should say. I am not an experimentalist, but I will add experimental physics to your tag list.

I think, not surprisingly, it might be a variety of sperm oil. If you really need to know more , Ken Reindel runs Ken's Clock clinc kensclockclinic.com and it looks like he knows, if anybody does, as he is a specialist for that era.