Ford Mustang Owner's Manual page 234

NOTE: If the vehicle has been operated for an
extended period at high speeds or in
city traffic during hot weather, or
pulling a trailer, the vehicle should be
turned off for about 30 minutes to
allow the fluid to cool before checking.
Adding Automatic Transmission Fluid
Before adding any fluid, be sure that the correct
type will be used. Use only MERCON fluid.
Add fluid in 1/2 pint (.25L) increments through
the filler tube to bring the level to the correct
area on the dipstick. DO NOT OVERFILL. If the
level is above the top hole on the dipstick,
excess fluid should be removed by a qualified
technician.
NOTE: Always dispose of used automotive
fluids in a responsible manner. Follow
your community's standards for
disposing of these types of fluids. Call
your local recycling center to find out
about recycling automotive fluids.
Detail of the automatic transmission dipstick
236