Um, yeah, you may want to check the brake fluid level in your master cylinder. But, if it's not that, it could be transmission fluid, but I would expect that to be more centered under the car. Also, I suppose it could be Power Steering fluid, but my Mustang doesn't have power steering so I'm not familar with the placement of its reservoir.
This is an easy one... Is the fluid green? if it is, it's coolant.. if not, its brake fluid.
Make sure it is oil and not brake fluid.
The drivers side of the engine
It is called the Windshield Washer Fluid Resevoir.
drivers side of engine ( V6 / V8 )
The PCV valve on 1992 ford mustang 5.0 is on the drivers side.
Under the dash on the drivers side
On a 1996 Ford Mustang : I believe the fuel pump inertia switch is behind the trunk liner , on the drivers side , near the drivers side rear tailight ( there is an access hole )
heater core
Under the dash on the drivers side
The drivers side of the engine is bank 2
Firing Orders; (4.6L - 5.4L) ************* Firewall ************ (Passenger Side) 4 * 8 (Drivers Side) (Passenger Side) 3 * 7 (Drivers Side) (Passenger Side) 2 * 6 (Drivers Side) (Passenger Side) 1 * 5 (Drivers Side) ************ Radiator *************