What type of oil did Millikan use for his experiment?
Millikan used vacuum pump oil for his experiment.
In 1906, Millikan and his graduate student Harvey Fletcher started the oil drop experiments. Fletcher did all the experimental work.
Millikan had tried various types of droplets. J.J. Thomson had used water droplets in his earlier experiments, so that was their first attempt. But the heat of the light source caused the small droplets to evaporate within about two seconds.
Millikan and Fletcher discussed possible other liquids such as mercury, glycerine, and oil. Fletcher bought some watch oil from the drugstore (a "Chemist's") and built a crude apparatus. It worked!
They refined the apparatus and eventually used vacuum pump oil. It had an extremely low vapour pressure and would not evaporate under the heat of the light source.
Here's a photo of the apparatus.
Compare this with teaching diagrams such as the one below.
Fletcher got his Ph.D. degree, and Millikan got a Nobel prize for this work.
Fletcher wrote a fascinating account of his work with Millikan. You can read it here.