FORD 2010 E-450 v.3 - Manual (Page 302)

Low fluid level
Do not drive the vehicle if the fluid level is at the bottom of the dipstick
and the ambient temperature is above 50°F (10°C).
Correct fluid level
The transmission fluid should be checked at normal operating
temperature 150°F-170°F (66°C-77°C) on a level surface. The normal
operating temperature can be reached after approximately 20 miles
(30 km) of driving.
You can check the fluid without driving if the ambient temperature is
above 50°F (10°C). However, if fluid is added at this time, an overfill
condition could result when the vehicle reaches normal operating
temperature.
ADD COLD HOT DO NOT ADD
Maintenance and Specifications
302
2010 Econoline (eco)
Owners Guide, 3rd Printing
USA (fus)