FORD 2006 F-150 v.3 - Manual (Page 307)

An overfill condition of transmission fluid may cause shift and/or
engagement concerns and/or possible damage.
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.
Checking and adding manual transmission fluid (if equipped)
1. Clean the filler plug.
2. Remove the filler plug and
inspect the fluid level.
2006 F-150 (f12)
Owners Guide (post-2002-fmt)
USA (fus)
Maintenance and Specifications
307