Most light aircraft run MIL-H-5606, mineral oil. only 2000 psi.
Metros run MIL-H-83282 at 2000 psi, i think there is some fire retardant properties. there's probably a few more on this one I imagine.
of course then we go to Skydrol.... Smallest airframes I know are Dash-8s and some bizjets. Other aircraft in their category are happy to use 5606.
K