Not refrigerant oil if the AC/Defroster is working. I am thinking clutch
fluid, if a manual tranny. Easy enough to tell; look at the reservoir. I
don't particularly suspect the brakes because the booster should stop the
fluid from getting into the passenger compartment, but while you are under
there a peek at all the fluid levels would be nice.

If it is the clutch fluid, you need a new clutch master cylinder. My
experience is that the master and slave cylinders usually go out within a
few months (sometimes weeks) of each other, so a pre-emptive replacement of
the slave would be a good idea.

If not a manual tranny, I'm stumped. Even a pinhole in the evaporator should
drain any oil leaked onto the ground through the evaporator drain.

Mike