Brake light comes on, fluid added because reservoir was low, some sort of fluid/oil dripping from under car…
It very well seems like a brake fluid leak, but coincidences sometimes lead one to chase a red herring.
The fluid feels oily and is not water?
You checked all fluid levels, cold, and brought them to full if necessary?
What I’d do, besides trying to examine everything by flashlight, mirror, and feel, is to drive carefully and continually monitor all fluid levels (coolant, transmission, clutch, power steering, brake, engine…) very frequently (each drive cycle). This leak could cause a problem in a very short amount of time (minutes, few miles).
An air conditioner failure can cause oil to leak, but you can’t pull a dipstick or visually check it.
If any fluids continue to drop then I’d rest assured that was my culprit system.
Use caution. Keep in mind that any minor leak can go major and with brake fluid it could be catastrophic and cause an out of control car.
Since it’s a 15 - 16 year-old car, leaks like this are not totally unexpected, especially if this car has operated where road salt is applied to roads in winter.
Take it to a mechanic or DIY, but be careful.