FORD 2015 E-450 - Manual (Page 180)

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.
Maintenance 179
2015 Econoline (eco)
Owners Guide gf, 1st Printing, June 2014
USA (fus)