FORD 2013 F-550 v.3 - Manual (Page 317)

Do not use supplemental transmission fluid additives, treatments or
cleaning agents. The use of these materials may affect transmission
operation and result in damage to internal transmission components.
TRANSFER CASE FLUID CHECK (IF EQUIPPED)
1. Clean the filler plug.
2. Remove the filler plug and
inspect the fluid level.
3. Add only enough fluid
through the filler opening so
that the fluid level is at the
bottom of the opening.
Use only fluid that meets Ford specifications. Refer to the Capacities
and Specifications section .
316 Maintenance
2013 F-250/350/450/550 (f23)
Owners Guide gf, 3rd Printing, December 2012
USA (fus)