What fluid is used in a door closer?

a light hydraulic oil, cannot say exactly what weight grade the oil is, but depending on the brand of closer, will depend on the weight and the color or the scent of the fluid.

the fluid best used in a door closer is an all weather (multi viscosity) fluid, this minimizes the need to adjust the door closer speed as the weather changes.

different brands call theirs a different name,

Norton for instance, they call their's norglide,

LCN they call their oil ultra liquid X